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©tje Ittttrermtij of ifflmtwaota 

MINNESOTA GEOLOGICAL SURVEY 
William H. Emmons, Director 

IN COOPERATION WITH THE UNITED STATES GEOLOGICAL SURVEY 

BULLETIN NO. 15 

PRELIMINARY REPORT ON THE GEOLOGY 
OF EAST CENTRAL MINNESOTA 
INCLUDING * 
THE CUYUNA IRON-ORE DISTRICT 

BY • - 

E. C. HARDER 
AND 

A. W. JOHNSTON 




MINNEAPOLIS 
The University of Minnesota 
1918 



Sty? UtttuMrBttij af MmntBvtn 



MINNESOTA GEOLOGICAL SURVEY 
William H. Emmons, Director 

IN COOPERATION WITH THE UNITED STATES GEOLOGICAL SURVEY 

BULLETIN NO. 15 

PRELIMINARY REPORT ON THE GEOLOGY 
OF EAST CENTRAL MINNESOTA 
INCLUDING 
THE CUYUNA IRON-ORE DISTRICT 

BY 

E. C. HARDER 
AND 

A. W. JOHNSTON 




MINNEAPOLIS 
The University of Minnesota 
1918 



CONTENTS 



Introduction I 

Acknowledgment 2 

Terminology 2 

General geology of Minnesota 4-15 

Rocks of east central Minnesota I 5~94 

Previous geologic work 15 

General distribution and lithology of the rocks 23 

Descriptions of rock outcrops 30 

Outcrops of silicic igneous rocks 30 

Southern Cass, Todd, and western Stearns counties .... 30 

Morrison County . ." 132 

Stearns, Benton, Sherburne, and western Mille Lacs 

counties 38 

Northern Mille Lacs, Kanabec, southern Aitkin, and 

western Pine counties 40 

Outcrops of subsilicic igneous rocks 49 

Todd County 49 

Morrison, Benton, and Stearns counties 50 

Aitkin, Carlton, and Pine counties 52 

Outcrops of slate, schist, and other metamorphic rocks 58 

Morrison and Benton counties 58 

Crow Wing, Cass, and Aitkin counties 63 

Pine, Carlton, and southern St. Louis counties 65 

Outcrops of sandstone and other clastic rocks 90 

Keweenawan sandstone and related rocks 90 • 

Cretaceous sediments 93 

The Cuyuna iron-ore district 94 _I 35 

Location of the district 94 

History of the district 96 

General geology of the district 107 

General statement 107 

Structure of the rocks in 

Lithology of the rocks 112 

Older metamorphosed rocks (Virginia slate) 112 

Later intrusive rocks 123 

Later volcanic and sedimentary rocks 124 

Occurrence of the iron-bearing rocks and iron ore 124 

Distribution of iron-bearing rocks and ore 124 

Occurrence and character of ore 129 

iii 



iv 



CONTENTS 



Geology of the principal mines 135-168 

North range mines 135 

Ferro mine 135 

Joan No. 3 mine 137 

Algoma mine 137 

Rowe mine 139 

Hillcrest mine 141 

Pennington mine 142 

Armour No. 1 mine 144 

I ronton mine 146 

Armour No. 2 mine 146 

Thompson mine 148 

Meacham mine 150 

Croft mine 150 

Cuyuna-Mille Lacs mine 152 

Mangan No. 1 mine 154 

Sultana mine 154 

Hopkins mine 156 

Joan mine 157 

Mahnomen mine 157 

Mangan No. 2 mine 159 

Kennedy mine 159 

South range mines 162 

Adams mine 162 

Hobart exploration shaft 163 

Wilcox mine 164 

Brainerd-Cuyuna mine 166 

Barrows mine 166 

Bibliography 169 



LIST OF ILLUSTRATIONS 

Plate I. Relief map of the Lake Superior district showing the 

location of the iron-ore districts 10 

II. Map of east central Minnesota showing the location 

of rock outcrops in pocket 

III. A. Granite outcrop at Meyers Quarry near Pierz, 

Morrison County, showing horizontal jointing 
B. Inclusions of dark gray granite in pink granite 

at Gravelville, Morrison County 34 

IV. A. Granite exposures on Rum River, south of 

Onamia, Mille Lacs County 
B. Granite exposures along Hay Creek, a branch of 

Snake River, Kanabec County 40 

V. A. Granite and mica schist outcrops at Lower Falls 
on Snake River, Kanabec County 
B. Granite exposures at Upper Falls on Snake River, 

Kanabec County 44 

VI. A. Outcrop of chloritic schist near the station at 
Randall, Morrison County 
B. Chloritic schist outcrops at Randall showing 

cleavage 58 

VII. A. Slate and phyllite outcrops on Mill Island at 
Little Falls, Morrison County 
B. Slate outcrop at Little Falls showing relation of 

cleavage to bedding 60 

VIII. A. Outcrop of slate and phyllite on Split Rock 
River, southwestern Carlton County, showing 
horizontal schistosity 
B. Outcrop of slate and phyllite along the railway 
northeast of Denham, Pine County, showing 

parallel jointing 66 

IX. A. Outcrops of slate along St. Louis River below 
Thomson Dam, Carlton County 
B. Outcrops of slate at the northern Pacific Railroad 
bridge over St. Louis River below Thomson 

Dam, Carlton County 86 

X. A. Calcareous concretions weathering out of slate at 
Carlton 

B. Small dike of diabase intruding slate at Carlton. . 89 



v 



LIST OF ILLUSTRATIONS 

XI. Map of the principal part of the Cuyuna iron-ore dis- 
trict showing distribution of known iron-bear- 
ing rocks and lines of maximum magnetic at- 
traction in pocket 

XII. A. Exploration by drilling in the Cuyuna district 

B. Starting excavation with steam shovel at the 

Mahnomen open-pit mine, near Ironton 96 

XIII. A. Removing overburden by hydraulic method at the 

Rowe mine, near Riverton 
B. Removing overburden by hydraulic method at the 

Hillcrest mine, near Ironton 100 

XIV. A. Sinking a concrete shaft at the Croft mine, near 

Crosby 

B. Iron-ore concentrating plant at the Rowe mine, 

near Riverton 102 

XV. A. Pennington-Armour No. 1 open-pit mine near 
Ironton 

B. Head- frame and trestle of Cuyuna-Mille Lacs 

mine, near Ironton 104 

XVI. A. Head-frame, concentrating plant, and part of 
open pit at Thompson mine, near Crosby 
B. Head-frame and power house of Adams mine, 

near Deerwood 106 

XVII. A. Ferruginous chert showing regular interbanding 
of chert and iron oxide 
B. Ferruginous chert showing irregular intermixing 

of chert and iron oxide 114 

XVIII. A. Wash ore showing porosity due to disintegration 
and removal of chert 
B. Minor plications in the iron-bearing rocks at the 

west end of the Rowe open pit 118 

XIX. Types of ore deposits in the Cuyuna iron-ore dis- 
trict 138 

XX. Map and sections showing the geology and workings 

of the Rowe mine in pocket 

XXI. Map and sections showing the geology and workings 
of the Pennington, Armour No. 1, and Armour 

No. 2 mines in pocket 

XXII. Map and sections showing the geology and workings 

of the Thompson mine in pocket 



PRELIMINARY REPORT ON THE GEOLOGY OF 
EAST CENTRAL MINNESOTA INCLUDING 
THE CUYUNA IRON -ORE DISTRICT 



By E. C. Harder and A. W. Johnston 
INTRODUCTION 

The geologic work in the Cuyuna iron-ore district is being done 
jointly by the Minnesota Geological Survey and the United States Geo- 
logical Survey. Since only a few outcrops of bed rock occur in the dis- 
trict or in the region adjacent to it, the study has been based largely on 
the results of exploration and mining work. Many drill cores and rec- 
ords of diamond drilling from various parts of the district have been 
examined, and in addition the occurrence and structure of the rocks as 
shown in the underground and open pit workings of various mines have 
been studied in detail, and detailed geologic and topographic maps have 
been made of several of the open pit mines. 

In order to gain a more comprehensive idea of the major structure 
of the rocks of east central Minnesota, and particularly to note if possible 
the relation of the rocks of the Cuyuna district to such major structure, 
a careful study of the rock outcrops of the region lying west, south, and 
east of the Cuyuna district was made. This included the detailed map- 
ping of most of the rock exposures lying nearest to the Cuyuna district 
in Cass, Todd, Morrison, Mille Lacs, Kanabec, Pine, Carlton, and Aitkin 
counties, and a more general study of the outlying exposures in these . 
counties and also in Stearns, Benton, and Sherburne counties. North of 
the Cuyuna district, in the region lying between it and the Mesabi dis- 
trict, no rock outcrops are known and the relation between the rock 
formations of these two districts will have to be determined by under- 
ground exploration. 

As the development work in the Cuyuna district is constantly in 
progress, it has been considered advisable to publish the present prelimi- 
nary report giving the general results of the investigations up to the 
present and to delay the publication of a more complete report pending 
the accumulation and study of further data. 

The present report is incomplete in many ways. The descriptions of 
the rock outcrops are very general. Thin sections for microscopic ex- 
amination of many of the rocks have not yet been made, and the exact 
mineral composition of these remains to be determined. In the descrip- 
tions, therefore, only the general characters and geologic relations of the 



2 



1XTR0DUCTI0N 



rocks are discussed, the details of texture and mineral composition being- 
only briefly mentioned. 

The structure and stratigraphy of the rocks of the Cuyuna district 
also remain to be determined. Information is as yet too incomplete to 
attempt arranging the various beds into a stratigraphic succession. It 
is a question even whether it will be possible to work out such a succes- 
sion for many years to come because of the general similarity of the 
rocks and the complex deformation which they have suffered. As re- 
gards the structure of the rocks more data are available and it is hoped 
that some progress may be made in this respect before the publication 
of the final report. 

The origin of the iron ore is only briefly mentioned, a more complete 
discussion being reserved for the final report. 

Accompanying the present report is a map showing the trend and 
location of the different belts of iron-bearing formation in the Cuyuna 
district. This is the first map which has been published giving in detail 
the surface distribution of the iron-bearing formation in the Cuyuna 
district, and it should be realized that the information on it is as yet far 
from complete. Doubtless this map will have to be revised, and the only 
reason for publishing it at the present time is the hope that it may assist 
in the exploration work which in this district is being conducted under 
many difficulties. 

ACKNOWLEDGMENT 

In the preparation of this report on the Cuyuna district the writers 
have had to depend very largely on the generosity of the exploration and 
mining companies in furnishing information and in allowing access to 
their records and files. Almost without exception the information has 
been willingly furnished and permission to examine drill cores, maps, 
and reports has been cheerfully given. In many instances companies 
and individuals have gone to considerable expense in furnishing blue 
prints and other materials to aid in the preparation of the report. For 
this cooperation the writers wish to express their sincere thanks. 

The writers are indebted to L. G. Ravicz, W. B. Lang, and E. A. 
Sweetman for assistance in the field. 

TERMINOLOGY 

The pre-Cambrian rocks of the Lake Superior region include a num- 
ber of rather uncommon lithological types. These have received dis- 
tinctive names which are in fairly general use throughout the region. 
Below are given brief explanations of some of the less well-known rock 
terms used in this paper as well as of more general rock terms whose 



TERMINOLOGY 



3 



usage in this paper has been restricted to certain definite types of rock. 

Iron-bearing formation is a term intended to include all the various 
ferruginous rocks which make up the iron-bearing beds, original as well 
as altered. 

Cherty iron carbonate or cherty siderite is a light to dark-colored, 
dense, fine-grained rock, usually banded and consisting of interlayered 
and intermixed chert and siderite. When much argillaceous material 
occurs in it, the term slaty or argillaceous iron carbonate is used. The 
iron in most of the iron-bearing formations of the Lake Superior region 
was probably originally deposited as carbonate. 

Greenalite rock is a dark green, granular, ferrous silicate rock occur- 
ring in the Mesabi district. Greenalite is believed to be the original rock 
from which the ferruginous chert and ore of the Mesabi district are 
derived. 

The term ferrous rock includes both ferrous carbonate and ferrous 
silicate rocks as well as mixtures of the two. 

Hematitic and limonitic cherts are variegated rocks, consisting of 
white, gray, or pink chert intermixed with more or less hematite or 
limonite. Commonly the chert and iron oxide occur interlaminated, 
forming a banded rock. Often, however, they are irregularly intermixed. 
Hematitic and limonitic cherts form the principal surface rocks of the 
iron-bearing formation. Ferruginous chert is a general term including 
both hematitic and limonitic cherts. Taconite is a granular ferruginous 
chert which occurs in the Mesabi district and is believed to be derived 
from the oxidation of greenalite rock. 

Hematitic and limonitic slates are thinly laminated dark red to brown 
argillaceous rocks heavily impregnated with hematite or limonite. They . 
show distinct banding and lamination and are of sedimentary origin. 
Ferruginous slate is a general term used to include both hematitic and 
limonitic slates. Paint rock is a term applied to soft altered phases of 
ferruginous slate. The term slate is perhaps a misnomer as applied to 
these rocks, since they only locally show slaty cleavage. They are 
rather indurated and metamorphosed shales for which the term argillite 
might be more properly used. The term slate, however, is in such gen- 
eral use for these rocks in the Lake Superior district that for the present 
it seems best to retain it. 

Jaspilite, or banded jasper, is a hard, dense, recrystallized, ferru- 
ginous chert. Hematite and jasper generally occur interbanded, the 
former frequently being in the specular form. 

Magnetitic slate, or, more properly, magnetitic argillite, is a dark, dis- 
tinctly laminated rock containing amphibole, magnetite, quartz, and other 
constituents usually somewhat segregated in parallel bands. It is a meta- 
morphosed phase of the original iron-bearing formation. 



4 



GEOLOGY OF EAST CENTRAL MLXXESOTA 



Amphibole-magnetite rock is similar to the magnetitic slate (or mag- 
netitic argillite) but shows coarser banding and crystallization and usually 
contains more quartz. 

Hematitic schist is a dark red, soft, homogeneous, schistose rock which 
is formed by alteration and impregnation with hematite from the green 
or gray chloritic schist which is commonly associated with the bands of 
iron-bearing formation. It is distinguished from ferruginous slate (or 
ferruginous argillite) in showing no distinct lamination and it is prob- 
ably of igneous origin. The term paint rock is applied to hematitic schist 
as well as to ferruginous slate. 

The term gneiss in this paper is applied to banded, metamorphosed, 
crystalline rocks which have approximately the composition of some deep- 
seated igneous rock. Usually they are siliceous in character. The dis- 
tinct banding or lamination is a requisite. 

Gneissoid granite is a granite which has suffered a certain amount of 
deformation or recrystallization so that the ferro-magnesian minerals 
have a somewhat parallel arrangement, giving the rock a foliated appear- 
ance. It does not show banding. 

Greenstone is a general term applied to dark green, massive, igneous 
rocks which have suffered alteration so that the original character is not 
obvious in the hand specimen. 

Green schist is a name given to dark green chloritic and amphibolitic 
rocks with schistose or slaty texture. Some are schistose greenstones, 
while others may be metamorphosed sediments. The more obviously 
laminated slaty green rocks of sedimentary origin are usually called 
green slates. 

Phyllite is finely crystalline quartz-mica schist. 

Slate is an indurated argillaceous rock showing slaty cleavage. It is 
usually very finely crystalline. 

The term argillite is used in this paper to denote indurated argillace- 
ous rocks which have no slaty cleavage, in contradistinction to the term 
slate. This usage, however, is not strictly adhered to because of the 
general use into which the term slate has come for all indurated, argil- 
laceous rocks in the Lake Superior region. 

GENERAL GEOLOGY OF MINNESOTA 

The basal rocks of Minnesota consist of a complex of granite, gneiss, 
and greenstone of Archean age with some metamorphosed sediments. 
Lying on these and folded into them are Huronian quartzite, conglom- 
erate, slate, schist, and iron-bearing formation, with locally crystalline 
limestone. These older rocks throughout much of the eastern part of 



GENERAL GEOLOGY OF MINNESOTA 



5 



the state are unconformably overlain by rocks of Keweenawan age, con- 
sisting in the north of flows and intrusive sheets of igneous rocks, both 
silicic and subsilicic, and in the south of red clastic sediments associated 
with detrital igneous rocks. The Keweenawan rocks also have suffered 
considerable deformation locally, but not to such a degree as the Huronian 
rocks. Keweenawan and Huronian silicic and subsilicic igneous rocks 
intrude older rocks as dikes, bosses, and sheets. 

The pre-Cambrian rocks form the bed rock underneath the glacial 
drift throughout the northeastern part of the state and for a consider- 
able distance west of the Mississippi River. In the southwestern part 
of the state also, several large patches of pre-Cambrian rocks are known 
to lie directly underneath the glacial drift. In the southeastern part of 
the state, however, the pre-Cambrian rocks are overlain by more or less 
flat-lying sediments of Paleozoic age, while throughout most of the 
western part of the state flat-lying, slightly consolidated, Cretaceous sedi- 
ments occur overlying the pre-Cambrian rocks. The glacial drift forms 
a mantle covering the different bed rocks practically throughout the state. 

The accompanying sections show the general relations between the 
rock strata in the southern and northeastern parts of the state. 

General Section of Rocks in Southern Minnesota and Western Wisconsin 1 



Quaternary 

Recent Alluvium 

Pleistocene Glacial till 

Cretaceous 

Benton shale (soft blue shale and unconsolidated sandstone) ) 
Dakota sandstone (white sandstone, shale, and red clay) j 

Devonian (limestone and sandstone) 

Ordovician 

Maquoketa shale (shale, dolomite, and argillaceous sandstone) 
Galena limestone 

Decorah shale y (limestone and shale) 

Platteville limestone J 

St. Peter sandstone (white or yellow sandstone, with some shale) 



Approximate 
Maximum 

Thickness 

575 feet 

500 feet 
100 feet 

100 feet 

350 feet 

200 feet 



1 Hall, C. W., Meinzer, O. E., and Fuller, M. L., Geology and underground waters of 
southern Minnesota: U. S. Geol. Survey Water Supply Paper 256, pp. 31-49. 1911. 

Hall, C. W., The gneisses, gabbro-schists, and associated rocks of southwestern Minnesota: 
U. S. Geol. Survey Bull. 157. 1899. 

Weidman, S., and Schultz, A. R., The underground and surface water supplies of Wis- 
consin: Wis. Geol. and Nat. Hist. Survey Bull. 35, pp. 25-43. 191 5- 

Winchell, N. H., Upham, W., and Harrington, M. W., (Geology of the southern counties 
of Minnesota), Geol. and Nat. Hist. Survey of Minnesota, Geol. of Minnesota, vols. I and II. 
1884, 1888. 



6 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Prairie du Chien group ("Lower Magnesian") 

Shakopee dolomite (yellow, buff, pink, or red dolomite, with 

sandstone lenses) 
Oneota dolomite (buff to reddish dolomite) 
Cambrian 

Upper Cambrian 

Jordan sandstone (coarse-grained white sandstone) 

St. Lawrence formation (dolomite, shale, and sandstone) 

Franconia sandstone (fine-grained white or yellow sandstone, 

with fossils and some greensand) 
Dresbach sandstone (heavily bedded coarse sandstone) 
Eau Claire shale (interbedded shale and shaly sandstone) 
Mt. Simon sandstone (coarse sandstone) 
Algonkian( ?) 

Keweena\van( ?) 

Red clastic series (red sandstone and shale, with volcanic 
clastic rocks) 

Algonkian 
Huronian 

Sioux quartzite (red quartzite and pipestone) 
Crystalline schist (hornblende and biotite schists) 

Archean 

Granite, gneiss, and schist 



75 feet 
200 feet 



50 feet 

60 feet 

160 feet 

100 feet 

250 feet 

250 feet 



2,250 feet 



Unknown 
Unknown 

Unknown 



General Section of Rocks 



Northeastern Minnesota and Adjacent Parts of 
Wisconsin 2 



Quaternary 

Recent Alluvium 

Pleistocene Glacial till 

Cretaceous 

Ferruginous conglomerate 
Algonkian 

Keweenawan 

Upper Keweenawan 

Bayfield group (In older reports mistaken for Cambrian. 
Lake Superior sandstone) 
Chequamegon sandstone (red and white quartz sand- 
stone) 

Devils Island sandstone (pink and white sandstone) 
Orienta sandstone (red and white sandstone) 



Average 
Maximum 

Thickness 



400 feet 
Small 



1,000 feet 
300 feet 
3,000 feet 



2 Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior region: U. S. Geol. 
Survey Mon. 52, pp. 118-224, 370-380. 191 1. 

Thwaites, F. T., Sandstones of the Wisconsin coast of Lake Superior: Wis. Geol. and Nat. 
Hist. Survey, Bull. 25. 19 12. 

Winchell, N. H., Upham, W., Todd, J. E., and Grant, U. S. (Geology of the central and 
northern counties of Minnesota), Geol. and Nat. Hist. Survey of Minn., Geol. of Minn., 2 and 
4. 1888 and 1899- 



GENERAL GEOLOGY OF MINNESOTA 



7 



Oronto group 

Amnicon formation (red and green shale and arkose) 5,000 feet 
Eileen sandstone (red and white feldspathic sandstone) 2,000 feet 
Freda sandstone (red arkose sandstone) 12,000 feet 

Nonesuch shale (black shale) 300 feet 

"Outer" conglomerate 1,200 feet 

Middle Keweenawan 

Diabase sills and laccoliths Varying 
Diabase amygdaloid, melaphyr, quartz porphyrite, and 
felsite extrusives, with interlayered conglomerate 
in upper part 4,000-20,000 feet 

Red rock, intrusive into Duluth gabbro and older rocks 

Duluth gabbro, laccolith at base of middle 

Keweenawan 9,500-25,000 feet+ 

Lower Keweenawan 

Puckwunge conglomerate 100-400 feet 

Huronian 

Upper Huronian (Animikie group) 

Virginia ("St. Louis") and Rove slates (argillaceous, 
siliceous, and carbonaceous slate, with lenses of 
quartzite, grit, graywacke, limestone, and iron-bear- 
ing formation) Unknown 
Biwabik and Gunflint formations (iron bearing) 620 feet 

Pokegama quartzite 200 feet 

Lower-middle Huronian 

Giants Range and other granites 

Knife Lake slate Unknown Slate and gray- 

Agawa formation (iron bearing) 0-50 feet J- wacke of Mesabi 

Ogishke conglomerate 0-2000 feet J Range 3,000-5,000 feet 

Archean 

Laurentian 

Granite and gneiss 
Keewatin 

Soudan formation (iron bearing) 1 Greenstone, green schist, 

I slate, and graywacke of 
Ely greenstone J Lake of the Woods region 

As shown by these sections, the rocks of Minnesota consist of a great 
series of pre-Cambrian plutonic, volcanic, metamorphic, and sedimentary- 
rocks overlain in places by a comparatively thin veneer of Paleozoic and 
Mesozoic rocks and more generally by a thin but wide-spread blanket of 
glacial deposits. 

The complex of pre-Cambrian metamorphic rocks and associated in- 
trusives is comparatively near the surface in the southwestern, central, 
and northeastern parts of the state, and surface outcrops are abundant in 
many places. In the northwestern part also these rocks are at no great 
depth, although outcrops are less common. In the southeastern part, 
however, a marked depression in their surface occurs in the form of a 
northeast-southwest basin, the bottom of which is several thousand feet 



8 



GEOLOGY OF EAST CENTRAL MINNESOTA 



below the general elevation of the pre-Cambrian rock surface in other 
parts of the state. 3 It is in this basin that the Keweenawan red clastic 
rocks occur, as well as the overlying Paleozoic sediments. 

A basin probably existed over part of this area before the Paleozoic 
sediments were deposited, but some sinking has taken place since that 
time. The original depression is believed to have been caused by defor- 
mational forces resulting in a tilting to the southeast. The relief of the 
pre-Cambrian surface before deposition of the Paleozoic sediments was 
very marked and the present approximately level pre-Cambrian rock 
surface in the northern and western parts of the state is due to subse- 
quent peneplanation. 4 Only locally in the southwestern part and beneath 
the sedimentary rocks in the southeastern part is the old uneven bed rock 
surface preserved. 

In southern Minnesota the principal pre-Cambrian rocks are granite, 
gneiss, Sioux quartzite, crystalline schists, and probably the red clastic 
series. Granite and gneiss are abundantly exposed along Minnesota 
River and at several localities in the region to the south. Elsewhere they 
are generally overlain by a thin layer of Cretaceous sediments. Hall 5 
states that wherever the crystalline rocks have been encountered under- 
neath the Cretaceous strata in drilling they have been found to be pro- 
foundly decomposed. Directly underneath the Cretaceous strata there 
is usually a layer of white clay varying in thickness up to 50 feet, which 
grades downward into decomposed granite or gneiss of yellow, red, or 
gray color. The Cretaceous itself consists principally of blue and red 
shale with some fine white quartz sandstone, products of decomposition 
of the underlying crystalline rocks. This decomposition indicates a long 
period of erosion previous to the deposition of the Cretaceous beds which 
probably extended through the Paleozoic and most of the Mesozoic era. 
That the residual material accumulated to such depths would indicate 
that the land surface during at least the latter part of the erosion period 
was low and comparatively level, though at the beginning of the Paleozoic 
there was probably considerable relief, as has already been stated. 

The Sioux quartzite is exposed at a number of places in southwestern 
Minnesota and probably forms the bed rock in most of the southwestern 
corner of the state. It consists largely of red quartzite, but locally thin 
layers of red slate (pipestone) occur in it. While the quartzite is hard 
and indurated and has suffered some folding, ripple marks and cross- 
bedding are still visible in it. 6 Its relation to the granite, gneiss, and 



8 Hall, C. W., Meinzer, O. E., and Fuller, M. L., op. cit. pp. 32-33. 

* Ibid., p. 33. 

5 Ibid., p. 49. 

« Ibid., p. 32. 



GEXERAL GEOLOGY OF MINNESOTA 



9 



crystalline schists on the one hand and to the red clastic series on the 
other hand is not known. The Sioux quartzite may lie unconformable 
above the crystalline rocks, or the crystalline rocks may be in part in- 
trusive into it. Its relation to the red clastic series also is not determined. 
Although apparently much more indurated and of somewhat different 
composition, it is not impossible that it may be of the same age as the 
red clastic rocks. Winchell and Upham 7 believed that the Sioux quart- 
zite, the red clastic series, and the supposed upper Keweenawan sediments 
south of Lake Superior (the latter formerly believed to be the same 
as the upper Cambrian Lake Superior sandstone of Michigan), are all 
of lower Cambrian age. Hall 8 on the other hand states that the red 
clastic rocks are the southward continuation of the Keweenawan of the 
Lake Superior region but that the Sioux quartzite is not related to them. 
In general it has been held by geologists that the Sioux quartzite is prob- 
ably of Huronian age. 

Hornblende and biotite schist which are believed to be the southw T est- 
ward continuation of the Huronian schist and slate which occur in east 
central Minnesota, have been encountered in well-drilling in the western 
part of the state north of Minnesota River. 

The red clastic series of southern Minnesota is only known 
from deep drilling. Outcrops of it are wanting, the beds being 
everywhere covered either by drift or by a considerable thickness 
of Paleozoic sediments ranging in age from Upper Cambrian to 
Devonian. The red clastic beds consist principally of red sand- 
stone and shale, but conglomerate beds also occur in it and vol- 
canic clastic material is found locally. The finer sediments are found 
mainly in the upper part. The series apparently lies conformably be- * 
neath the known Upper Cambrian and unconformably above the pre- 
Cambrian crystalline rocks. Their age has not been determined, but be- 
cause of their lithological similarity to the sandstone occurring on the 
south shore of Lake Superior in Wisconsin, which has recently been re- 
ferred to the upper Keweenawan, 9 it is supposed that they are equivalent. 
The continuation of one into the other, however, has not been proven 
because of the scarcity of exposures in east central Minnesota and because 
of the general similarity between the red clastic rocks and the rocks 
forming the lower portion of the Upper Cambrian. 

In northern Minnesota outcrops of pre-Cambrian rocks are abundant 
in many places, and a much more complex series of them has been found 



7 Winche)l, N. H., and Upham, W., Geol. and Nat. Hist. Survey of Minn., Geol. of Minn. 
vol. i, pp. 422, 424, and 537. 1882. 

8 Hall, C W., Meinzer, O. E. t and Fuller, M. L., op. ext., p. 48. 
»Thwaites, F. T., op. cit. 



10 



GEOLOGY OF EAST CENTRAL MINNESOTA 



to exist than occurs in the southern part of the state. There are Archean 
metamorphic and igneous rocks, Huronian intrusive rocks, and metamor- 
phosed sediments, and Keweenawan intrusive and extrusive rocks and 
sediments. These rocks have been studied in considerable detail in sev- 
eral districts, particularly in the Mesabi and Vermilion iron-ore districts 
and along the north shore of Lake Superior. 10 They have been shown 
to be of great complexity both in structure and distribution. The Archean 
rocks are most abundant along the northern boundary of the state and 
to the southward Huronian and Keweenawan rocks appear with increas- 
ing prominence. 

The Archean rocks are principally Laurentian granite and gneiss and 
Keewatin greenstone. The greenstone is either massive or schistose and 
locally contains metamorphosed sediments, such as iron-bearing forma- 
tion in the Vermilion district and slate and graywacke in the Lake of the 
Woods region. In some localities the greenstone has suffered so little 
deformation that original structures are still clearly visible, while else- 
where it has been metamorphosed to such an extent as to be transformed 
into green chloritic schist. Large and small masses of Laurentian granite 
and gneiss occur intruded into the greenstone in the form of batholiths, 
bosses, and dikes. Much of the granite has been rendered gneissoid by 
later deformation and recrystallization so that now great areas in northern 
Minnesota are underlain by coarse gneiss and all gradations from gran- 
ite to gneiss occur. In composition the Laurentian intrusives vary from 
granite to syenite and texturally both granitic and porphyritic types occur. 
Special phases of the intrusives are felsitic rocks, such as rhyolitic and 
trachytic porphyries. 

Metamorphosed sedimentary rocks of lower-middle Huronian age are 
not very extensively distributed in northern Minnesota. In the Ver- 
milion district this series is represented by the Ogishke conglomerate, 
iron-bearing Agawa formation, and Knife Lake slate. The conglomerate 
occurs at the base, and is overlain locally by Agawa iron-bearing forma- 
tion, and above this is the Knife Lake slate. 

The Ogishke conglomerate varies extremely in thickness in different 
parts of the district. It has been thoroughly indurated and locally pro- 
nounced metamorphism has resulted in the development of schistose 
phases while intense folding has brought about complex relations between 
it and the underlying Archean. The materials composing the conglom- 



50 Van Hise, C. R., and Leith, C. K., op. cit., pp. 1 18-210. 

Leith, C. K., The Mesabi iron-bearing district of Minnesota. U. S. Geo!. Survey, Mott. 
43- 1903. 

Clements, J. M., The Vermilion iron-bearing district of Minnesota. U. S. Geo!. Survey, 
Mon. 45. 1903. 



PLATE I 




LIEF MAP OF THE LAKE SUPERIOR DISTRICT SHOWING THE LOCATION OF THE IRON-ORE DISTRICTS, AFTER VAN HISE AND LEITH 

SCALE, I INCH TO ABOUT 60 MILES 



GENERAL GEOLOGY OF MINNESOTA 



ii 



erate have been derived from the underlying Keewatin and Laurentian 
rocks, pebbles of greenstone, granite, porphyry and jasper being found 
in it. 

The Knife Lake slate covers considerable areas and is of great thick- 
ness. It includes many different phases such as argillaceous slate, biotitic, 
amphibolitic, quartzitic, and sericitic schists, graywacke, and conglomer- 
ate. Its metamorphism has been intense, and complex folding has re- 
sulted in the development of secondary structures, such as cleavage and 
schistosity, throughout the formation. 

The iron-bearing Agawa formation occurs as thin lenses or layers 
below the Knife Lake slate and above the Ogishke conglomerate. It 
consists of ferruginous chert, jasper, and other iron-bearing rocks inter- 
bedded with layers of slate. 

In the Mesabi district the lower-middle Huronian is represented by a 
thick series of slate, graywacke, and conglomerate. These rocks occur in 
local areas, being complexly infolded into the underlying Keewatin green- 
stone. Metamorphism has resulted in the development of cleavage and 
fissility, and in the formation of secondary minerals, such as sericite, bio- 
tite, and chlorite. The conglomeratic phases particularly show the effects 
of metamorphism in the elongation of pebbles and the recrystallization 
and alteration of minerals. 

In general the lower-middle Huronian metamorphosed sediments have 
a complex distribution, occurring in local irregular areas with intricate 
structural relation with the underlying Archean. In some localities they 
appear to have suffered practically as much metamorphism as the Kee- 
watin greenstone itself. 

The lower-middle Huronian sediments are intruded by the Giants 
Range granite, principally a pink to gray hornblende granite, low in 
quartz, which occupies a large area north of the Mesabi district. Silicic 
intrusives of lower-middle Huronian age are abundant in many parts of 
the Lake Superior region. They are difficult to distinguish from the 
Laurentian intrusives, however, and it is only in the more recent detailed 
geologic work in which the relations between the intrusives and the asso- 
ciated metamorphosed sediments have been carefully studied that the 
separation has been possible. 

On the eroded surface of the Archean and lower-middle Huronian 
rocks lie the much less deformed upper Huronian or Animikie beds. The 
Animikie group in northern and central Minnesota consists in general of a 
thick slate formation at the base of which are comparatively thin layers 
of iron-bearing formation and quartzite. The principal area of Animikie 
rocks in Minnesota begins at the international boundary near Gunflint 
Lake and runs southwestward as a narrow, discontinuous strip to the 



12 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Mesabi district where it widens and spreads out over most of east central 
Minnesota as far southward as Mille Lacs Lake. In the Mesabi district 
the Animikie rocks lie on the south slope of the Giants Range and have 
a gentle dip to the southeast. The Pokegama quartzite occurs at the base 
of the group. It is a thin but fairly continuous bed consisting of vitreous 
quartzite, which locally gives place to micaceous quartzitic argillite and 
elsewhere is conglomeratic. 

The iron-bearing Biwabik formation which lies above the quartzite is 
somewhat thicker than the latter and is continuous throughout the length 
of the Mesabi district. It contains various common phases of the iron- 
bearing formation such as ferruginous chert of the form known as taco- 
nite, ferruginous slate, iron ore, greenalite rock, and quartz-amphibole- 
magnetite rock. The principal part of the iron-bearing formation consists 
of taconite. The ferruginous slate occurs in this as layers and lenses 
some of which are quite continuous and may be traced from mine to 
mine. 11 Quartz-amphibole-magnetite rock is found at the eastern end of 
the Mesabi district and has resulted from the metamorphism of the orig- 
inal iron-bearing formation by the Keweenawan gabbro intrusion. Iron 
ore occurs as local concentrations in the taconite, while remnants of 
greenalite rock are found in places where the iron-bearing formation has 
been protected from oxidation. 

The Virginia slate, which overlies the Biwabik formation, consists of 
argillaceous and carbonaceous slate with local layers of siliceous slate and 
graywacke. It forms the main upper portion of the Animikie group 
throughout east central Minnesota, having a great but unknown thick- 
ness. In the Mesabi district this rock is usually soft and but slightly 
metamorphosed, although mica occurs in it locally. It shows no secondary 
cleavage, being really an argillite or indurated shale. South of the Me- 
sabi district, however, along St. Louis River near Cloquet and Carlton, 
the slate, which in the past has been called "St. Louis slate," but which 
Van Hise and Leith referred to as the Virginia slate, has suffered meta- 
morphism and deformation to a marked degree, as have also the slate and 
garnetiferous and staurolitic schist exposed along Mississippi River 
near Little Falls. 

The schists, slates, and related rocks of the Cuyuna district and the ad- 
jacent region are also correlated with the Virginia slate by Van Hise and 
Leith and are believed to be of upper Huronian or Animikie age. They 
are principally sericitic, biotitic, and chloritic schist, slate, and phyllite 
with locally lenses of black, carbonaceous slate and some quartzite and 



u Wolff, J. F., Recent geologic developments on the Mesabi iron range, Minnesota. Bull. 
Amer. Inst. Min. Engrs., Oct., 1916, pp. 1763-1787. 



GENERAL GEOLOGY OF MINNESOTA 



13 



quartzitic schist. A prominent member of the series is the Deerwood 
iron-bearing member, which occurs as layers and lenses interbedded with 
the schist. The rocks of the Cuyuna district have been complexly folded 
into a series of close anticlines and synclines striking northeast-southwest. 
The beds usually have very steep dips either to the southeast or north- 
west, while the pitch of the folds is usually very low so that frequently 
the crests of the anticlines are horizontal for long distances. The rocks 
of the Cuyuna district will be described in more detail later. 

Overlying the older rocks in northeastern and eastern Minnesota, and 
intruded into them, are rocks of Keweenawan age. The oldest of these 
are sedimentary beds supposed to be of lower Keweenawan age, which 
occur unconformably overlying the Animikie rocks at several places in 
northeastern Minnesota. Among them is the Puckwunge conglomerate 
found along St. Louis River west of Duluth. 

The principal Keweenawan rocks of northern and eastern Minne- 
sota are intrusive and extrusive igneous rocks of middle Keweenawan 
age. They form a large area extending along the north shore of Lake 
Superior from Duluth northeasterly into Canada, and a smaller area along 
the upper St. Croix River south of Lake Superior. The extrusive rocks 
occurring along the north shore of Lake Superior are mainly diabase 
amygdaloids, but important layers of felsite and quartz porphyrite occur 
locally. The principal intrusive rock is the Duluth gabbro which forms 
an extensive laccolith at or near the base of the lavas. Smaller sills and 
sheets of diabase, such as the Beaver Bay laccolith, are found intrusive 
in the volcanic rocks at several horizons. The rocks in the area along 
St. Croix River are mainly extrusive rocks of amygdaloid type, asso- 
ciated with crystalline phases, such as diabase, and with beds of tuff and ■ 
volcanic breccia. 12 Thin layers of conglomerate occur locally. 

Sediments of upper Keweenawan age are abundant along the south 
shore of Lake Superior extending from Superior eastward into northern 
Michigan. They consist mainly of red and white sandstone and arkose 
with increasing shale in the lower part and a thick conglomerate at the 
base. The upper portion of the series, recently classed as the Bayfield 
group of the upper Keweenawan, 13 has generally been called Lake Supe- 
rior sandstone and included in the Cambrian, 14 while the lower portion, 
known as the Oronto group, has always been classed as upper Keweena- 
wan. Certain of the formations in the Bayfield group, extend south- 
westward into Minnesota, but their extent and distribution are not 



12 Grout, F. F., Contribution to the petrography of the Keweenawan: Jour. Geology, vol. 
18, pp. 633-657- 1910. 

"Thwaites, F. T., op. cit. 

"Van Hise, C. R., and Leith, C. K., op. cit., pp. 378-379, 415. 616. 



14 



GEOLOGY OF EAST CENTRAL MIXXESOTA 



known, due to lack of exposures. At some point east or southeast 
of Mille Lacs Lake they give place to the lower members of the fos- 
siliferous Upper Cambrian series extending northward from south- 
eastern Minnesota and western Wisconsin. This boundary has not yet 
been located. It seems probable that the red clastic series, which under- 
lies the Upper Cambrian rocks in southeastern Minnesota, emerges from 
beneath the latter on going northward and comes to the bed rock surface 
in east central Minnesota. In this case it might be directly continuous 
with the Bayfield group extending southwestward from Lake Superior. 

The Paleozoic rocks of southeastern Minnesota range from Upper 
Cambrian to Devonian in age. 15 They consist in general of interlayered, 
comparatively thin beds of sandstone, limestone, and shale. The upper 
beds, such as the Devonian, are found only in the southeastern part of 
the state while successively older formations come to the bed-rock sur- 
face on going northward. Thus the Platteville limestone and St. Peter 
sandstone constitute the principal surface rocks in the region adjacent 
to St. Paul and Minneapolis, while the Oneota and Shakopee dolomites 
and the upper members of the Upper Cambrian form outcrops along 
St. Croix River below Taylor's Falls. Above Taylor's Falls various 
Upper Cambrian strata occur along St. Croix River continuing north- 
ward until they give place to Keweenawan trap rock. 

The Paleozoic beds are very uniform in character. For this reason 
and because of the abundance of fossils in them, even thin beds can be 
recognized over great areas. The lower portion of the Paleozoic section 
represents a fairly continuous period of deposition, although minor un- 
conformities may occur. 16 A marked break, however, is found in the 
upper part, for the Devonian strata rest unconformably on the eroded 
surface of the Ordovician, indicating an erosion period covering the 
Silurian and part of the Devonian. The uniformity in the lithology and 
general character of the Paleozoic beds over large areas presents a strong 
contrast to the pre-Cambrian rocks of the State, which as a rule are most 
irregular in their lithology and stratigraphy. 

The mantle of Cretaceous rocks, although thin, is fairly continuous 
throughout the western part of Minnesota, while isolated areas of various 
sizes occur also in the eastern part. In the southern and western parts 
of the State these rocks consist typically of thick beds of plastic gray-blue 
shale associated with thin beds of white sandstone. 17 The latter occur 
mainly in the upper and lower parts of the series. Locally beds of red 
clav are found. 



16 Hall, C. W., Meinzer, O. E., and Fuller, M. L., of. at., pp. 3*"4£- 
" Ibid., p. 33- 

17 Ibid., p. 42- 



ROCKS OF EAST CENTRAL MINNESOTA 



*5 



In the east central and northern parts of the State isolated patches of 
Cretaceous sediments are found in many places. In the Mesabi and 
Cuyuna districts they consist mainly of ferruginous conglomerate asso- 
ciated with argillaceous material. Elsewhere, however, as along 
Mississippi and Sauk rivers in the central part, beds of slightly consoli- 
dated calcareous clay and sand are found. 

ROCKS OF EAST CENTRAL MINNESOTA 

PREVIOUS GEOLOGIC WORK 

The first geological investigations of any considerable importance 
made in east central Minnesota were those of R. D. Irving and E. T. 
Sweet in connection with similar work in northern Wisconsin. 18 The 
work was done between 1873 an< ^ l &79 an d consisted in an examination 
of Keweenawan and Huronian rocks adjacent to the Wisconsin bound- 
ary. In a report 19 published in 1880, Irving stated his belief that the 
slates occurring along St. Louis River were to be correlated with 
the iron-bearing rocks of the Penokee region, and that they probably 
occupied the northern limb of a syncline of which the rocks of the 
Penokee district formed the southern limb. At this time no subdivision 
had as yet been made of the Huronian rocks and the schists, slates, and 
other metamorphosed sediments of northern and central Minnesota and 
neighboring regions were all grouped together as Huronian to distinguish 
them from the overlying Keweenawan and from the granite and gneiss, 
these three being the only subdivisions of the pre-Cambrian recognized 
at that time. 

The investigations of the United States Geological Survey began in 
this region about 1880 under the direction of R. D. Irving assisted by 
C. R. Van Hise and W. N. Merriam. Brief reconnaissance surveys 
were made in the early 8o's in central and northern Minnesota and the 
first general geologic map of the Lake Superior region to be published 
by the United States Geological Survey appeared in 1883. 20 On this 
map the Huronian metamorphosed sedimentary rocks of northern and 
eastern Minnesota, including those of the Vermilion district as well as 



18 Irving, R. D., The geological structure of northern Wisconsin: Geol. of Wis., vol. Ill, 
Survey of 1873-79, p. 18. 1880. 

Sweet, E. T., Geology of the western Lake Superior district: Geol. of Wis., vol. Ill, 
Survey of 1873-79, p. 334 et seq., 1880. 

19 Irving, R. D., op. cit. 

20 Irving, R. D., The copper-bearing rocks of Lake Superior: U. S. Geol. Survey Third Ann. 
Rept., 1881-1882, p. 162 and PI. III. 1883. 

Irving, R. D., The copper-bearing rocks of Lake Superior: U. S. Geol. Survey Mon. 5, p. 
384 and PI. I. 1883. 



i6 



GEOLOGY OF EAST CEXTRAL MIXXESOTA 



those occurring along the Giants Range or Mesabi Range, are all grouped 
with the Animikie rocks of the north shore of Lake Superior, and the 
slates and schists occurring along St. Louis and Kettle rivers are 
connected directly with the Animikie rocks on the north shore. The 
Animikie rocks at this time apparently were regarded as a somewhat less 
metamorphosed phase of the other Huronian rocks without recognition 
of difference in age. Apparently it was not then recognized that the less 
metamorphosed rocks of the Mesabi district are younger than the highly 
metamorphosed slates and associated rocks of the Vermilion district. 

In a later report accompanied by a general map, 21 Irving correlates 
the "St. Louis" slate with the slates and schists occurring along Mis- 
sissippi River in the vicinity of Little Falls, and on the basis of this cor- 
relation an area of Huronian metamorphosed rocks is shown in east 
central Minnesota extending from Duluth southwestward to Missis- 
sippi River, including the present Cuyuna district and the northern part 
of Mille Lacs Lake. It is shown as being bounded on the south and 
west by areas of granite the age of which is not given, and on the north 
by a drift-covered area in which the bed rock is unknown. The area 
of Huronian rocks of east central Minnesota is not directly connected 
with the Animikie area extending from the Giants Range northeastward 
along the north shore of Lake Superior as was done in the previous re- 
port, but the statement is made that they are probably equivalent to them. 

In 1888 Irving published a map of northeastern Minnesota 22 in 
which the Animikie rocks occurring along the Giants Range are mapped 
as extending southward into east central Minnesota without a break. 
On this map the metamorphosed sediments of the Vermilion district and 
of the international boundary are separated from the Animikie rocks of 
the north shore of Lake Superior and of the Giants Range because of their 
greater metamorphism. but it is believed nevertheless that they are equiv- 
alent in age, the difference in lithology being due to difference in the 
intensity of metamorphism. The granite and gneiss of northern and 
central Minnesota, which had hitherto been left unclassified, are placed 
in the Laurentian. and with them are grouped certain green schists and 
mica schists which were believed to be older than Huronian. The fol- 
lowing classification of the pre-Cambrian rocks is used. It is of interest 
because it is the forerunner of the classification used at present by the 
United States Geological Survey for Lake Superior rocks. 



n Irving, R. D., Preliminary paper on an investigation of the Archean formations of the 
northwestern states: I'. S. Geol. Suney Fifth Ann. Reft., 1S83-1884, pp. 196-107. 1885. 

- Irving, R. D., Classification of earlv Cambrian and pre-Cambrian formations: U. S. Geol. 
Survey Seventh Ann. Rept., 1883-1886, pp. 417-423- 1888. 



ROCKS OF EAST CENTRAL MINNESOTA 



17 



Cambrian 

Potsdam sandstone 
Keweenawan 

Volcanic flows and sediments 

Basal intrusive gabbro 
Huronian 

Animikie series 

Vermilion Lake iron-bearing series 
Laurentian 

Green schists 
Mica schists 
Granite 

In 1892, C. R. Van Hise published a report on the pre-Cambrian rocks 
of North America, which contains a section on the geology of the Lake 
Superior region. 23 A general table shows the correlation of rocks in 
different parts of the region, the following subdivisions being used : 
Algonkian 

Keweenawan 
Upper Huronian 
Lower Huronian 
Archean 

Mareniscan 
Laurentian 

This is a further stage in the development of the classification of 
Lake Superior pre-Cambrian rocks. As regards the application to the 
rocks of northern and central Minnesota, it is similar to those previously 
used except that the term Archean is substituted for Laurentian, the 
latter term being restricted to intrusive granite and gneiss, while a new 
term, Mareniscan, is used to cover the older metamorphic rocks, such as 
green schists and mica schists. The younger metamorphosed sediments 
(Huronian) which are grouped together with the Keweenawan under 
the general subdivision Algonkian, are definitely separated into two series, 
the earlier (lower Huronian) including the metamorphosed sediments 
of the Vermilion region, which had formerly been but indefinitely sep- 
arated from the Animikie, and the later (upper Huronian) including the 
Animikie rocks and "St. Louis" slate. A map accompanying this discus- 
sion shows the general geology of the Lake Superior region and is very 
similar to the maps previously published. On it the "St. Louis" slate is 
shown as extending across east central Minnesota, including the present 



23 Van Hise, C. R., Correlation papers: Archean and Algonkian: U. S. Geol. Survey Bull. 
86, pp. 51-208. 1892. 



i8 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Cuyuna district and Little Falls region, being bounded on the west and 
south by granite classified as Archean. 

While this work was being done by the United States Geological Sur- 
vey, N. H. Winchell, Warren Upham, and others of the Geological and 
Natural History Survey of Minnesota, were also engaged in extensive 
geological investigations in central and northern Minnesota. The work 
of the Minnesota survey began in 1872, but during the early years it 
was confined largely to the southern part of the state. Later, however, 
extensive investigations were made on the crystalline and metamorphic 
rocks of central and northern Minnesota. The work consisted chiefly 
in the mapping of rock outcrops 24 in this region, but attempts were also 
made by Winchell to correlate the various rock formations with similar 
rocks in the neighboring states, in Canada, and in New England. 25 

This resulted in the adoption of a somewhat different nomenclature 
from that used by geologists of the United States Geological Survey. 
The table on the following page shows the classification of Lake Superior 
rocks used by the Geological and Natural History Survey of Minnesota, 
as compared with that used later in reports of the United States Geo- 
logical Survey. 26 

This classification was partly the outcome of work by Winchell in 
northern and eastern Minnesota, and partly of work by A. C. Lawson in 
the Lake of the Woods and Rainy Lake regions for the Geological and 
Natural History Survey of Canada. 27 Not only does the nomenclature 
itself differ from that used by the United States Geological Survey, but 
the position assigned to many of the rock formations in the stratigraphic 
series is radically different. Perhaps the most fundamental difference 



24 Winchell, N. H., Upham, Warren, and others, Geo!, and Nat. Hist. Survey of Minn., 
Geol. of Minn., vols. 2, 4, and 6. 1 882-1 901. 

Upham, Warren, Notes of rock outcrops in central Minnesota: Geol. and Nat. Hist. Survey 
of Minn. Eleventh Ann. Rept., pp. 86-136 and PI. I. 1882. 

25 Winchell, N. H., The crystalline rocks of the northwest: Geol. and Nat. Hist. Survey 
of Minn. Thirteenth Ann. Rept., pp. 124-140. 1884. 

The crystalline rocks of Minnesota: Geol. and Nat. Hist. Survey of Minn. 

Seventeenth Ann. Rept., pp. 5-74. 1888. 

, The crystalline rocks: Geol. and Nat. Hist. Survey of Minn. Twentieth Ann. 

Rept., pp. 1-28. 1891. 

26 Winchell, N. H., Note on the age of the rocks of the Mesabi and Vermilion iron dis- 
tricts: Geol. and Nat. Hist. Survey of Minn., Eleventh Ann. Rept., pp. 168-170. 1882. 

, Pre-Silurian rocks of Minnesota: Geol. and Nat. Hist. Survey of Minn., 

Twenty-first Ann. Rept., Table opp. p. 4. 1892. 

Winchell, N. H., Upham, Warren, and others, Geol. and Nat. Hist. Survey of Minn., 
Geol. of Minn., vol. 6, Geological map — frontispiece. 1900-1901. 

27 Lawson, A. C, Report on the geology of the Lake of the Woods region, with special 
reference to the Keewatin (Huronian?) belt of Archean rocks: Geol. and Nat. Hist. Survey of 
Canada, Ann. Rept. for 1885, vol. 1 (new series) pp. 5-15 1. CC. 

Report on the geology of the Rainy Lake region: Geol. and Nat. Hist. Survey 

of Canada, Ann. Rept. for 1887-1888, vol. 3 (new series) pp. 1-196 F. 

, Lake Superior stratigraphy: Am. Geol., vol. 7, pp. 320-327. 1891. 



ROCKS OF EAST CENTRAL MINNESOTA 



19 



in the two classifications is that the entire "Taconic series" is classed as 
Paleozoic by Winchell, being regarded as analogous to the so-called 
Taconic rocks of New England. The rest of the crystalline and 
metamorphic rocks are classed as Archean, no Algonkian being rec- 
ognized. There is no distinction made by Winchell and Lawson between 
the rocks of the lower Huronian and those of the Keewatin, all being 
grouped together under the term Keewatin, while the Vermilion series 
included the Coutchiching rocks of Lawson, as well as various other 
crystalline schists of northern and central Minnesota. In this group, 
which Winchell at first designated as Montalban, were placed the gar- 
netiferous and staurolitic schist found along Mississippi River near 
Little Falls, as well as certain gneisses occurring in Morrison County east 
of Little Falls. The Animikie of Winchell included the original Animikie 
rocks on the north shore of Lake Superior, the iron-bearing series of the 
Mesabi district, and the slates occurring along St. Louis River, but 
did not include the slates and schists of the Kettle River region, these 
being classed by Winchell as Keewatin on account of their greater meta- 
morphism. 



Minnesota Geological and Natural History 
Survey 

Paleozoic 

Upper Cambrian 
Taconic 

Keweenawan (Nipigon) 
Upper 

Brown sandstone 
Lower 

Traps and amygdaloidal 
rocks 

Gabbro and anorthosite 
Animikie 

Metamorphosed sediments, 
including Mesabi series, 
Sioux quartzite, and 
others 

Archean 

Ontarian 

Keewatin 

Sericitic and green schists, slate, 
graywacke, and other meta- 
morphosed rocks of Ver- 
milion district 
Vermilion (Montalban, Coutchiching) 
Holocrystalline hornblende and 
mica schists 

Laurentian 

Gneiss 



United States Geological 

Survey 
Paleozoic 

Upper Cambrian 
Algonkian 

Keweenawan 
Upper 

Middle 



Lower 
Huronian 



/ 



/I 



Upper 

Middle 
Lower 



/ Archean 
/ f <J Laurentian 

/ / 

— -{ Kewatin 



/ 



20 



GEOLOGY OF EAST CENTRAL MIXXESOTA 



The slate, graywacke, and schist of St. Louis and Kettle rivers, and 
of the Little Falls region, which were all believed by geologists of the 
United States Geological Survey to be of approximately the same age, 
were not so regarded by Winchell, who grouped them under three dif- 
ferent subdivisions, the more highly metamorphosed rocks being placed 
in the Archean and the less highly metamorphosed ones in the Taconic. 

In 1894, J. E. Spurr published an article on the so-called 'Thomson" 
or "St. Louis" slate 28 in which he brings forth various arguments why 
these slates should be classed as "Keewatin (lower Huronian)" rather 
than as Animikie (upper Huronian) as had been done hitherto. He bases 
his arguments chiefly on the lithological similarity of the "St. Louis" slate 
with what he calls "Keewatin (lower Huronian)" rocks of the Mesabi 
district, and on the fact that secondary structures, such as cleavage, seem 
to indicate that they suffered deformation by the same dynamic forces. 
In his paper the slates and schists of the Kettle River region and of the 
Little Falls region are also classed as "Keewatin (lower Huronian)" 
and are believed to be directly continuous with the "St. Louis" slate. 

The first general report of the United States Geological Survey on 
the iron ores of the Lake Superior region was published in 1 901. 29 It 
contains a short discussion of the general stratigraphy of the Lake Supe- 
rior region, and more detailed descriptions of the geology of the Ver- 
milion and Mesabi districts. The classification of the rocks is similar to 
that in present use, with the exception that the middle Huronian had not 
yet been recognized. The general terms Archean and Algonkian are 
used. The Archean includes the greenstones, various schists, and asso- 
ciated iron-bearing formation as well as the older granites and gneisses. 
The Algonkian is divided into (1) lower Huronian. which includes vari- 
ous highly metamorphosed sediments of the Vermilion and Mesabi dis- 
tricts, (2) upper Huronian, which includes the iron-bearing rocks of the 
Mesabi district and of the Gunflint area, and (3) Keweenawan, under 
which are grouped various later igneous and sedimentary rocks. It is 
recognized that not all the silicic igneous rocks of the region are Archean 
in age, but that later intrusive rocks occur such as the lower Huronian 
granite of the Giants Range. No special reference is made to the slates 
and schists of eastern and central Minnesota. 

Various monographs of the United States Geological Survey describ- 
ing the iron ores of the Lake Superior region, which were published 
between 1892 and 1900, contain discussions of the general geology of the 



28 Spurr. J. E., The stratigraphic position of the Thomson slates: Amer. Jour. Sci., 3d 
ser., vol. 148, pp. 159-166. 1894. 

29 Van Hise, C. R., The iron-ore deposits of the Lake Superior region: U. S. Geol. Survey, 
Twenty-first Ann. Rept., pt. 3, pp. 307-434. 1901. 



ROCKS OF EAST CENTRAL MINNESOTA 



21 



region, but the first ones which dealt directly with the geology of Minne- 
sota were the monographs on the Vermilion and Mesabi iron-ore dis- 
tricts 30 which appeared in 1903. 

In the monograph on the Vermilion district, the various intensely 
metamorphosed Huronian rocks of northern Minnesota are classified 
under subdivisions of the lower Huronian, while the greenstone, green 
schist, and associated iron-bearing formation are placed in the Keewatin 
series of the Archean. Silicic intrusives of two ages are recognized, those 
north of the Vermilion district being mapped as Laurentian, while those 
south of the district are mapped as lower Huronian. 31 The crystalline 
and metamorphic rocks of northern Minnesota were thus definitely cor- 
related with similar rocks in northern Wisconsin and Michigan described 
in the earlier monographs. 

In the monograph on the Mesabi district, the iron-bearing rocks are 
classed as upper Huronian while certain metamorphosed slates and gray- 
wackes included in the Archean by Winchell, Grant, and Spurr 32 of the 
Minnesota survey, are included in the lower Huronian. The Giants 
Range granite is classified as lower Huronian, while a supposedly later 
granite, the Embarrass granite, believed to be intrusive into the upper 
Huronian, is included in the Keweenawan. 

An interesting discussion in the report deals with a possible connec- 
tion between the upper Huronian rocks of the Mesabi district of Minne- 
sota and those of the Penokee-Gogebic district of Michigan and Wis- 
consin. 33 It is accompanied by a map showing the hypothetical course 
of a possible iron-bearing belt extending southwestward from the western 
end of the Mesabi district for some distance and then turning and run- 
ning eastward through the present Cuyuna district almost to Duluth, at 
which point it again turns, and after running southwestward for a con- 
siderable distance, makes several more turns and then connects with 
the western end of the Penokee-Gogebic iron-bearing belt. The existence 
of such a connecting belt was based on the similarity in lithology and 
succession of the rocks of the Mesabi district with those of the Penokee- 
Gogebic district, while its general trend was suggested by the structure 
of the Lake Superior syclinorium and by a number of scattered rock 



80 Clements, J. M., The Vermilion iron-bearing district of Minnesota: U. S. Geol. Survey, 
Mon. 45. 1903. 

Leith, C. K., The Mesabi iron-bearing district of Minnesota: U. S. Geol. Survey, Mon. 
43- 1903. 

31 Clements, J. M., op. ext., pp. 33 and 275-395. 

82 Grant, U. S., and Winchell, N. H., (Detailed geology of areas in northeastern Minne- 
sota), Geol. and Nat. Hist. Surrey of Minn., Geol. of Minn., vol. 4, 1896-1898, pp. 346-580. 1899. 

Spurr, J. E., The iron-bearing rocks of the Mesabi range in Minnesota: Geol. and Nat. 
Hist. Survey of Minn., Bull. no. 10. 1894. 

83 Leith, C. K., op. cit., pp. 202-204. 



22 



GEOLOGY OF EAST CENTRAL MINNESOTA 



outcrops. Its course through the Cuyuna district was determined by 
certain quartzite exposures near Dam Lake three miles south of the town 
of Kimberly, which are identical in lithology with the Pokegama quart- 
zite of the Mesabi district. On the shore of Long Lake, southwest of 
Dam Lake, are outcrops of subsilicic igneous rocks which were presumed 
to be equivalent to the Archean greenstone of the Mesabi district and 
to underlie this quartzite. Variations in magnetic attraction had been 
reported at various times from localities in Crow Wing and Aitkin coun- 
ties, and these were considered as additional evidence for drawing the 
supposed iron-bearing belt through the Cuyuna district. Leith at this 
time, following Spurr, 34 supposed that the slate outcrops at Carlton and 
Cloquet and at Little Falls were of lower Huronian age, and believed 
that they were southwest of the iron-bearing belt and within the area 
of the underlying lower Huronian and Archean. In the latter were in- 
cluded also the granite outcrops occurring at Staples, St. Cloud, and at 
various places west, south, and southeast of Mille Lacs Lake. 

In more recent reports on the geology of the Lake Superior district 
by Van Hise and Leith, 35 prepared since the discovery of iron-bearing 
formation and iron ore in the Cuyuna district, the slates and schists of 
east central Minnesota are all classified as upper Huronian and are cor- 
related with the Virginia slate of the Mesabi district. The slates occur- 
ring along St. Louis and Kettle rivers, the schists and associated 
metamorphosed rocks of the Cuyuna district, and the slates and schists 
found along Mississippi River near Little Falls are all believed to be 
more or less equivalent. The iron-bearing formation of the Cuyuna dis- 
trict is called the Deerwood iron-bearing member and is described as 
occurring in beds and lenses in the lower part of the Virginia slate. The 
base of the slate, however, is not known anywhere in eastern or central 
Minnesota, so that, the underlying formations, whatever they may be, 
have not been discovered. The granite, gneiss, and other igneous rocks 
of central Minnesota which had previously been regarded as lower 
Huronian or Archean, are in this report classified as Keweenawan on 
account of their supposed intrusive relation to the upper Huronian slates. 

Outside of east central Minnesota but few changes were made in the 
classification of the rocks. The Giants Range granite, however, and 
various highly metamorphosed sediments of the Mesabi and Vermilion 
districts which had hitherto been included in the lower Huronian are re- 
classified as lower-middle Huronian to correspond with the lower and 
middle Huronian of northern Michigan. 

84 Spurr, J. E., The stratigraphic position of the Thomson slates: Amer. Jour. Set., 3d ser., 
vol. 148, pp. 159-166. 1894. 

36 Van Hise, C. R., and Leith, C. K., The geology of the Lake Superior region: U. S. Geol. 
Survey, Mon. 52, pp. 211-224. 191 1. 

, Pre-Cambrian rocks of North America: U. S. Geol. 

Survey Bull. 360, pp. 196-252. 1909- 



ROCKS OF EAST CENTRAL MINNESOTA 



23 



GENERAL DISTRIBUTION AND LITHOLOGY OF THE ROCKS 

A heavy mantle of glacial till covers the older rocks throughout east 
central Minnesota, obscuring their distribution and their relations to 
each other. Only here and there isolated masses of the underlying rocks 
project up through it. Such projections are mainly of harder rock, which 
formed hills and ridges above the general level of the surface on which 
the glacial material was deposited. Locally also the rivers have cut down 
through the glacial drift and exposed the underlying rock. Outcrops of 
bed rock are closely spaced in parts of Stearns, Sherburne, Benton, Mor- 
rison, Mille Lacs, Kanabec, Pine, and Carlton 36 counties, but in other 
counties, among them Crow Wing, Todd, Cass, and Aitkin, rock ex- 
posures are either entirely lacking or are relatively rare. 

In the east central part of Minnesota, as has been mentioned, the 
bed rock consists mainly of ancient metamorphosed pre-Cambrian rocks. 
Paleozoic and Mesozoic rocks do not cover large areas. Probably they 
never extended over the entire area and where they were deposited they 
were nearly everywhere removed by erosion before the advent of the 
glacial epoch. Some Paleozoic rocks, however, occur in the area traversed 
by the lower courses of St. Croix and Rum rivers, and outliers of Meso- 
zoic sediments are present in many places in the region traversed by 
Mississippi River. 

The boundary between the pre-Cambrian rocks on the west and north- 
west and the Paleozoic rocks on the east and southeast, trends a little 
east of north through the south central part of the State, and continuing 
northward crosses Mississippi River into the southeastern part of Sher- 
burne County. From there the boundary continues in the same general 
northeast direction to the upper St. Croix River, but its location has not 
been definitely established. As already stated, the Keweenawan sedi- 
ments which extend southwestward from the west end of Lake Superior 
are very similar lithologically to the sediments forming the lower part 
of the Paleozoic, and have not as yet been separated from them in this 
region. Fossiliferous Paleozoic sediments have been identified as far 
north as the southern part of Pine County, while sediments classed as 
upper Keweenawan 37 are definitely known as far south as the southern 
part of Carlton County, and probably they extend some distance farther. 
Numerous outcrops of red, pink, and white sandstone occur in eastern 
Kanabec and western Pine counties, but since fossils have not been found 



36 Hall, C. W., Keewatin area of eastern and central Minnesota: Geol. Soc. America Bull. 
vol. 12, pp. 343-376. 1901. 

Upham, Warren, Notes on rock outcrops in central Minnesota: Geol. and Nat. Hist. Sur- 
rey of Minn., Eleventh Ann. Kept., pp. 86-136 and PI. I. 1882. 

37 Thwaites, F. T., Sandstones of the Wisconsin coast of Lake Superior: Wis. Geol. and 
Nat. Hist. Survey Bull. 25. 191 2. 



24 



GEOLOGY OF EAST CEXTRAL MIXXESOTA 



at any of these localities it has not been possible to determine the age of 
these rocks. There is thus in east central Minnesota an area of sand- 
stone of undetermined age between the northernmost known Paleozoic 
sediments and the southernmost known sediments of supposed upper 
Keweenawan age. 

The Paleozoic sediments in the southern part and the Keweenawan 
sediments and igneous rocks in the northern part of east central Minne- 
sota form the eastern margin of an extensive area of pre-Cambrian 
metamorphosed sediments and igneous rocks. On the west, these older, 
metamorphosed rocks are covered by nearly flat-lying, partly consolidated 
Cretaceous sediments. The eastern limit of the main Cretaceous area is 
west of Mississippi River. The boundary is a very irregular one, patches 
of Cretaceous rock occurring in many places within the area of older 
rocks both east and west of Mississippi River. 

The following is a tentative section of the rocks of east central Minne- 
sota, essentially that employed by the United States Geological Survey. 33 

Section of Rocks for East Central Minnesota 

Quaternary 
Recent 

Alluvium 
Pleistocene 

Late Wisconsin till 
Early Wisconsin till 
Cretaceous 

Yellow and gray clay and sand, in part calcareous 

Ferruginous conglomerate 
Cambrian 

Upper Cambrian 

Sandstone and shale 
Algonkian 

Keweenawan 

Intrusives. Granite, syenite, diorite, diabase, and gabbro 
Pink and red sandstone and arkose 

Volcanic flows. Amygdaloid, tuff, breccia, and diabase 
Huronian 

Upper Huronian (Animikie group) 

Virginia slate (including "St. Louis" slate and slates and schists of 
Cuyuna Range and Little Falls area) — quartzitic, carbonaceous, 
micaceous, chloritic, amphibolitic, garnetiferous, and staurolitic 
schist and slate with lenses of graywacke, quartzite, grit, limestone, 
and Deerwood iron-bearing member, with which are associated 
masses of metamorphosed subsilicic igneous rocks 



M Van Hise, C. R.. and Leith, C. K., The geology of the Lake Superior region: U. S. Geol. 
Survey Mon. 52, pp. 595-626 and 211-224. 191 1. 



ROCKS OF EAST CENTRAL MINNESOTA 



25 



Lower-middle Huronian (possibly present) 

Granite (equivalent to Giants Range granite) 
Archean (possibly present) 

Granite and gneiss 

No rocks of undoubted Archean age occur in east central Minnesota. 
There are, however, in Cass, Todd, Morrison, Stearns, Sherburne, Ben- 
ton, Mille Lacs, Kanabec, Aitkin, and Pine counties, numerous outcrops 
of granite and gneiss the age of which has not been definitely determined. 
Some of the granite in the eastern part of this area is intruded into schist 
believed to be of upper Huronian age and is therefore, post-upper 
Huronian. Other granites, especially those in the western part of the 
area, give some indication of being lithologically and areally related to the 
lower-middle Huronian Giants Range granite of the Mesabi district. 

Besides these, however, there are many outcrops of coarse gneissoid 
granite, banded gneiss, and augen-gneiss which resemble the Archean 
rocks more closely than any of the later intrusives. Banded gneisses and 
gneissoid granites are prominently exposed on Hillman Creek and Skunk 
River in Morrison County, on Rum River in Mille Lacs County, and 
along the Minneapolis, St. Paul, and Sault Ste. Marie Railway west of 
Denbam and elsewhere in Pine County. Augen-gneiss occurs in a series 
of outcrops near McGrath and Arthyde in Aitkin County, and near Den- 
ham in Pine County. 

Outcrops of subsilicic igneous rocks are also found in east central 
Minnesota, but less abundantly than those of silicic rocks, and most of 
them represent intrusions into the upper Huronian schists. They are, 
therefore, probably related to the subsilicic intrusives of Keweenawan 
age which occur so abundantly north and south of Lake Superior. In . 
a number of places, however, outcrops of subsilicic rock occur of which 
the relationship is not clear. This is especially true of the diabase and 
diorite exposures occurring along the shores of Long Lake in Aitkin 
County and at various localities in Carlton County. Winchell 39 cor- 
related the rocks cropping out at Long Lake with the Keewatin green- 
stone of the Vermilion district in northern Minnesota, while Van Hise 
and Leith 10 have classified them as Keweenawan. Recent exploration in 
the Cuyuna district, however, has shown that similar rocks occur both 
intrusive into and interbedded with upper Huronian schists in this dis- 
trict. It has been found also that many of these rocks have suffered con- 
siderable dynamic metamorphism. Therefore it seems probable that 
subsilicic rocks of upper Huronian as well as Keweenawan age occur in 



38 Winchell, N. H., The Cuyuna iron range: Econ. Geology, vol. 2, pp. 565-571. 1907. 
"Van Hise, C. R., and Leith, C. K., op. cit., PI. I. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



east central Minnesota, but the existence of Archean subsilicic igneous 
rocks in this region seems doubtful. 

There is thus considerable uncertainty as to whether Archean rocks 
are found in east central Minnesota. Since, however, the age of most 
of the silicic igneous rocks in this region has not been determined and 
since rocks similar to those which exist here are known to occur in the 
areas of Archean rocks in the southwestern and northern parts of the 
state, it does not seem improbable that some of the silicic intrusive rocks 
of east central Minnesota are of Archean age. 

No rocks of known lower or middle Huronian age have been found 
in east central Minnesota. The granite outcrops of Cass and Todd coun- 
ties, which will be described later, present certain similarities, however, 
to the Giants Range granite of the Mesabi district and may be continu- 
ous with it. This view is supported by the fact th'at granite has been 
encountered in drilling at several localities between the western end of 
the Mesabi district and the Cass and Todd county outcrops. Among the 
other outcrops of silicic igneous rocks of undetermined age in east central 
Minnesota many may be of lower-middle Huronian age as well. 

The most interesting rock exposures of east central Minnesota as 
regards structure and correlation are those of metamorphosed sediments 
which occur in various parts of the region and which have been classed 
as upper Huronian or Animikie in age. The most important of these 
are the slate, graywacke, phyllite, and mica schist outcrops occurring along 
St. Louis and Kettle rivers in Carlton and northern Pine counties, the 
quartzite outcrops at Dam Lake, south of Kimberly, Aitkin County, and 
the slate, phyllite, and mica, staurolite and chlorite schist outcrops occur- 
ring along Mississippi River and a few of its branches in Morrison County. 
Drilling and mining operations in the Cuyuna iron-ore district have shown 
the presence of an extensive area of metamorphic rocks, covered by gla- 
cial drift in central Crow Wing and western Aitkin counties. Most of 
the metamorphosed rocks which have been found in east central Minne- 
sota are of sedimentary origin, but some of them have a structure, dis- 
tribution, and general character that strongly suggest an igneous origin. 

The outcrops in Carlton and Pine counties consist mainly of slate, 
graywacke, phyllite, and mica and hornblende schists. Slate, graywacke, 
and phyllite occur in the northern part, at Carlton and Cloquet, while to 
the south phyllite and coarse mica schist occur near Moose Lake and 
northeast of Denham, and hornblende schist is found at several localities 
along Kettle River west of Moose Lake. South of Denham, near the 
southern edge of the main region of metamorphosed sediments, there 
occurs a series of interbedded mica and amphibole schist, quartzite, grit, 
and crystalline limestone which is bounded on the south by an extensive 



ROCKS OF EAST CENTRAL MINNESOTA 



27 



area of gneiss. The strike of the sedimentary structures throughout this 
region is approximately east and west, while the dips vary on each side of 
vertical. 

The detailed work on the outcrops of Pine and Carlton counties is not 
yet completed and the general structure has not been definitely deter- 
mined. The rocks appear to lie in a series of east-west trending folds 
with horizontal or slightly eastward or westward pitching axial lines. 
Probably the predominating pitch of the folds is to the east indicating 
that to the west rocks of lower stratigraphic horizons may be expected 
to occur. The largest folds found up to the present have a width of 
several miles and numerous minor folds down to fine crenulations occur 
on their limbs. 

The quartzite outcrops at Dam Lake consist of light gray to pink 
quartzite. They show no bedding or secondary structures by which 
their relation to the other rocks of central Minnesota might be determined. 

The schist and slate outcrops along Mississippi River grade from 
finely micaceous slate and phyllite near Little Falls to coarsely crystal- 
line, garnetiferous and staurolitic biotite schist in the southern part of 
Morrison County. Where it is possible to distinguish the bedding as at 
the mouth of Little Elk River, and at Little Falls, the strike of the rocks 
is in general northeast-southwest. The dips vary due to undulatory 
folding. 

Northwest of Little Falls near Randall several areas of outcrops con- 
sisting of chloritic schist occur not far from Little Elk River, 8 to 10 
miles above its mouth. These rocks have the same strike as the slate 
beds of Little Falls, and apparently form a belt running parallel to and 
northwest of the slate belt. 

No connection has been demonstrated between the slate and mica schist 
near Little Falls and that occurring near Carlton and Moose Lake. As 
they are found approximately along the same general line of strike, how- 
ever, it is possible that they may be continuous. This view is somewhat 
strengthened by the fact that, in recent exploration work in the dry una 
district, the chloritic schist occurring at Randall has been shown to con- 
tinue northeastward through Crow Wing County and for some distance 
into Aitkin County. If the slate and mica schist belt follows along parallel 
to the chloritic schist belt and southeast of it, as might be expected, this 
would bring it to within a comparatively short distance of the exposures 
at Moose Lake and Carlton. Drilling in T. 45N., R. 24W., about 10 
miles east of the north end of Mille Lacs Lake has shown the presence 
of mica schist in this intervening area. 

The relation of the slate, phyllite, and schist of east central Minne- 
sota to the Virginia slate in the Mesabi district has not been determined. 



28 



GEOLOGY OF EAST CENTRAL MINNESOTA 



As regards the metamorphism and deformation which they have suffered, 
the contrast between them is great. While the Virginia slate has a gentle 
dip to the south and shows little or no secondary cleavage, being practi- 
cally an indurated shale, the slate and schist of east central Minnesota 
have been closely folded and highly metamorphosed so that cleavage is 
developed throughout, and locally the mineral constituents have been 
coarsely recrystallized. Van Hise and Leith, however, believe that the 
Virginia slate continues southward from the Mesabi district into east cen- 
tral Minnesota where it is represented by these metamorphosed phases. 
But since no rock outcrops are known for at least 40 miles south of the 
Mesabi district, certain correlation is difficult. It is not impossible that 
the schist and slate of east central Minnesota are of middle or lower 
Huronian age. In their general character and in the deformation which 
they have suffered they resemble the older rocks very closely. 

The metamorphosed sediments have been intruded by both silicic and 
subsilicic igneous rocks in many parts of east central Minnesota. Sub- 
silicic rocks, such as gabbro, diabase and diorite, are especially common in 
the western portion. Silicic rocks such as biotite granite, occur more 
abundantly in the eastern portion. The igneous rocks are in the form of 
dikes, bosses, and irregular masses. Since the metamorphosed sediments 
are believed to be of upper Huronian age, these intrusives have been 
designated as Keweenawan. They are believed to be the vents through 
which were poured out the lavas that at present occupy the valley of the 
upper St. Croix River and which in former times probably extended 
westward over a considerable portion of east central Minnesota. 

There are a number of localities in east central Minnesota where sub- 
silicic igneous rocks which have suffered considerable dynamic metamor- 
phism occur. These, as has been mentioned, are believed to be older 
than Keweenawan, being associated with the upper Huronian metamor- 
phosed sediments and probably representing contemporaneous flows or 
slightly younger intrusive masses. They have been deformed and meta- 
morphosed with the associated sediments. 

The Keweenawan rocks occuring along the upper St. Croix River are 
mainly of the subsilicic extrusive type, including diabase, diabase amygda- 
loid, tuff, and breccia. 41 Interlayered with these are local beds of con- 
glomerate. A general characteristic of the lava beds is that each bed 



41 Grout, F. F., Contributions to the petrography of the Keweenawan: Jour. Geology, vol. 
18, pp 633-657. 1910. 

Hall, C. W., Keweenawan area of eastern Minnesota: Bull. Geol. Soc. America, vol. 12, 

PP- 3I3-342- 1901. 

Bcrkey, C. P., Geology of the St. Croix dalles: Amcr. Geol., vol. 20, pp. 345-383, 1897, 
and vol. 21, pp. 139-155 and 270-294, 189S. 



ROCKS OF EAST CENTRAL MINNESOTA 



29 



consists of three texturally distinct phases of rock. The lower few 
inches usually consist of dense, finely crystalline, finely shattered rock. 
Above this is the main mass of the flow which is usually medium coarsely 
crystalline, distinctly ophitic in texture, and is free from amygdules or 
other cavities. The crystalline phase grades upwards into an amygda- 
loidal portion which represents the surface of the flow. Locally the upper 
portions of a flow may be tuffaceous or brecciated, while in other places 
important layers of volcanic clastic rocks are interbedded between suc- 
cessive flows. The coarseness of crystallization of the main mass of the 
flow depends largely on the thickness. Some of the more extensive flows, 
which range up to two hundred feet in thickness, are fairly coarsely 
crystalline. 

The conglomerate layers are usually thin and are very local in their 
occurrence. At several localities, however, beds as thick as one hundred 
feet have been reported. 42 

The St. Croix area of Keweenawan lavas is delimited on the west by 
a fault which extends northeastward toward Lake Superior, forming the 
boundary between the eastward dipping Keweenawan lavas, on the south- 
east, and the horizontally bedded, non-fossiliferous sandstone of sup- 
posed upper Keweenawan age, already mentioned, on the northwest. Be- 
cause of this fault it has been impossible to determine the relation which 
these two formations bear to each other. 

In many places in eastern Minnesota outcrops of horizontally bedded 
sandstone, designated as "Hinckley sandstone," "Potsdam sandstone," 
and "St. Croix sandstone," have been reported by geologists of the early 
Geological and Natural History Survey of Minnesota. 43 The outcrops 
designated as "Hinckley sandstone" or "Potsdam sandstone" are for the , 
most part non-fossiliferous, and, as has been mentioned, many of these 
probably are to be correlated with the Bayfield group of the upper 
Keweenawan. In the southern part of the region, however, especially 
along St. Croix River, there are numerous outcrops of fossiliferous sand- 
stone designated as "St. Croix sandstone," which are probably equivalent 



42 Hall, C. W., op. cit., pp. 326 and 329. 

43 Upham, Warren, The geology of Mille Lacs and Kanabec counties: Geol. and Nat. Hist. 
Survey of Minn., vol. 2 (Geology) 1882-1885, pp. 612-628. 1888. 

Upham, Warren, The geology of Chisago, Isanti, and Anoka counties: Geol. and Nat. Hist. 
Survey of Minn., vol. 2 (Geology) 1882-1885, pp. 399-425. 1888. 

Upham, Warren, The geology of Pine County: Geol. and Nat. Hist. Survey of Minn., 
vol. 2 (Geology) 1882-1885, pp. 629-645. 1888. 

Winchell, N. H., The geology of Carlton County: Geol. and Nat. Hist. Survey of Minn., 
vol. 4 (Geology) 1896-1898, pp. 1-24. 1899. 

Upham, Warren, The geology of Aitkin County: Geol. and Nat. Hist. Survey of Minn., 
vol. 4 (Geology) 1896-1898, pp. 25-54. *&99- 

Winchell, N. H., The geology of the southern portion of St. Louis County: Geol. and 
Nat. Hist. Survey of Minn., vol. 4 (Geology) 1896-1898, pp. 212-221. 1899. 



30 



GEOLOGY OF EAST CENTRAL MINNESOTA 



to certain of the lower formations of the Upper Cambrian. Fossiliferous 
sandstones are found along St. Croix River as far north as Rush Creek, 
a short distance below the mouth of Snake River. Similar flat-lying sedi- 
ments in which, however, fossils have not been found, occur for some 
distance north of this point, continuing beyond the mouth of Kettle River. 

There is thus along St. Croix River an area of flat-lying sandstone 
which toward the south contains fossils of Upper Cambrian age. This 
area is bounded on the west by steeply eastward dipping Keweenawan 
flows with interlayered conglomerate. Beyond the volcanic rocks, sep- 
arated from them by a fault, is another belt of flat-lying sandstone, which, 
however, is non-fossiliferous. It has generally been supposed that these 
two sandstones are of approximately the same age, and hence they have 
usually been mapped as one formation. More recent study, however, 
indicates that eventually a separation may be made between them, throw- 
ing the non-fossiliferous sandstone into the Keweenawan. 

Slightly consolidated Cretaceous sediments are known to occur at a 
number of localities in east central Minnesota. Exposures are found at 
a few places south of Little Falls along Mississippi River and west of it, 
while in the Cuyuna district these rocks have been encountered in drilling 
and mining operations. The exposures along Mississippi River consist 
principally of sandy and calcareous clay. Some portions are slightly con- 
solidated, while other portions are quite soft. Locally beds occur con- 
taining little ferruginous concretions. The Cretaceous rock encountered 
in exploration work in the Cuyuna district is principally ferruginous con- 
glomerate and shale. 

DESCRIPTIONS OF ROCK OUTCROPS 
OUTCROPS OF SILICIC IGNEOUS ROCKS 
SOUTHERN CASS, TODD, AND WESTERN STEARNS COUNTIES 

The westernmost outcrops of silicic igneous rocks in central Minne- 
sota are those occurring in the southwestern corner of Cass County, 
northeast of Staples, those occurring near Long Prairie River south of 
Staples in Todd County, and those occurring in the vicinity of Sauk Cen- 
ter and Melrose, and in Ashley township, in the western part of Stearns 
County. 

The exposures northeast of Staples are scattered over a prominent 
ridge north of Crow Wing River, and occupy an area about half a mile 
long. The principal rock of these exposures is a pink to greenish, light- 
colored, medium-grained hornblende granite. In some phases of it, 
quartz and feldspar are almost equally abundant, but in other phases 
quartz is exceedingly rare and the rock grades into syenite. In this 
respect it resembles the Giants Range granite of the Mesabi district. The 



ROCKS OF EAST CENTRAL MINNESOTA 



3i 



principal ferromagnesian mineral is green hornblende, but biotite is 
present in some phases. The granite is intruded by dikes of dark green 
rock of intermediate composition. The dikes are fine-grained along the 
contacts but are medium-grained in the center. They vary in thickness 
up to thirty or forty feet. Both the granite and the green dikes are 
strongly epidotized. 

The group of outcrops in Todd County is located in the west town- 
ship of Ward about 1 mile west of Long Prairie River, and about 5 miles 
northeast of Browerville. 44 There are several exposures. The rock is 
described by Upham as a medium-grained gray syenite consisting mainly 
of quartz, feldspar, and dark hornblende. It is somewhat similar to the 
rock found northeast of Staples and resembles it also in being epidotized. 

In the northwestern part of Stearns County three groups of outcrops 
occur near the towns of Sauk Center and Melrose, and in the township 
of Ashley. The outcrops in Ashley township consist of numerous ex- 
posures south of Ashley Creek. 45 The rock resembles syenite but has been 
strongly epidotized so that no hornblende or mica remains, and it consists 
mainly of feldspar and epidote. The Sauk Center outcrops are found 
a short distance southeast of the depot. Several varieties of rock occur, 
the principal one being a feldspathic reddish gneiss, laminated in a north- 
east-southwest direction. Associated with the gneiss are reddish syenite 
and masses of pink quartz-feldspar pegmatite. Several exposures of a 
hard, dark, granular hornblende feldspar rock occur, probably syenite. 
The outcrops at Melrose are within the village, south of Sauk River. 
They consist of hard, coarse, red syenite. In the western part of the 
village a well is said to have been sunk in a dark, coarsely crystalline 
hornblende rock. 46 

In the eastern part of Cass County, southeast of Leech Lake, drilling 
has shown the occurrence of granite in three localities along a line run- 
ning southwestward from the western end of the Mesabi district. This 
rock has been mapped by Van Hise and Leith 47 as Giants Range granite 
and is supposed to be continuous with that in the Mesabi district. On the 
other hand, the granite outcrops at Staples, as well as numerous other 
outcrops to the south and southeast, have been mapped by them as Ke- 
weenawan on the supposition that they are intrusive into the upper 
Huronian schist and slate. However, this intrusive relation has been defi- 
nitely established for only a few of the outcrops of silicic igneous rocks in 

"Upham, Warren, The geology of Wadena and Todd counties: Geol. and Nat. Hist. Sur- 
vey of Minn., Geol. of Minn., vol. 2, 1882-1885, pp. 568-569. 

45 Upham, Warren, The geology of Stearns County: Geol. and Nat. Hist. Survey of Minn., 
Geol. of Minn., vol. 2,- 1882-1885, pp. 452-454. 

46 Upham, Warren, op. cit. 

47 Van Hise, C. R., and Leith, C. K., The geology of the Lake Superior region: U. S. 
Geol. Survey Mon. 52, Plate I. 



22 



GEOLOGY OF EAST CENTRAL MINNESOTA 



central Minnesota, and since lithologically the Cass County and Todd 
County outcrops resemble the Giants Range granite it is not impossible 
that they may be of the same age, viz., lower-middle Huronian. 

MORRISON COUNTY 

In Morrison County outcrops of silicic igneous rocks are abundant, 
especially in the central and eastern part, east of Mississippi River. The 
principal exposures occur in an area approximately 18 miles long, east 
and west, and 12 miles wide north and south, but small isolated outcrops 
are found outside of this area. 

Several small exposures of medium to coarse-grained syenite are 
found on the east bank of Mississippi River in Little Falls, about a quar- 
ter of a mile below the dam and approximately opposite the lower part of 
the island which occurs in the river at this place. The rock consists of 
hornblende and feldspar, the former locally altered in part to epidote and 
the latter to zoisite, epidote, muscovite, and quartz. Some quartz and 
biotite occur in the rock also. Locally chlorite is found as an alteration 
product of biotite and hornblende. Apatite and titanite are accessory 
minerals. The occurrence of this igneous rock within a short distance of 
the slate and phyllite outcrops at the dam and on the island would 
seem to indicate that it is intrusive into the sedimentary rocks. No actual 
contacts, however, have been found. 

An important group of granite and gneiss outcrops is that occurring 
in the eastern part of Morrison County on Platte River, Skunk River, 
and Hillman Creek. In general these are of gray and pink biotite and 
hornblende granites, locally somewhat gneissoid in texture. A few out- 
crops of banded gneiss are found. 

The northernmost of these outcrops is a group of exposures occur- 
ring on both sides of the east-west township road between section 34, 
T. 42N., R. 31W., and section 3, T. 41 N., R. 31W. Two types of granite 
occur in these exposures, a medium fine-grained grayish brown or brown 
decomposed biotite granite, considerably epidotized, and a grayish pink 
or pink hornblende granite. The former has a fairly uniform grain, but 
the latter varies in texture from fine-grained to very coarse-grained, 
almost pegmatitic, and also shows porphyritic phases. The hornblende 
granite appears to be intrusive in the biotite granite. 

In the northwest corner of section 1, T. 41 N., R. 31W., and the 
southwest corner of section 36, T. 42N., R. 31 W., about i\ 2 miles east 
of the outcrops just described, and one mile north of Freedhem, there 
are a number of exposures of fine-grained, dark gray, biotite granite 
mixed with a pinkish, medium to coarse-grained granite which usually 
contains both biotite and hornblende. From the relations it appears that 
the pink granite is intrusive as dikes and irregular masses into the dark 



ROCKS OF EAST CENTRAL MINNESOTA 



33 



gray granite which is by far the more abundant. The pink granite con- 
tains much pink feldspar while the dark gray granite is rich in quartz and 
biotite. The rocks crop out in a swamp both north and south of the town- 
ship road. 

Near the center of section 6, T. 41 N., R. 30W., about iy 2 miles 
south of east from the outcrops just described, are a few scattered 
outcrops consisting principally of fine to medium-grained, dark gray bio- 
tite granite, rich in quartz, which is intruded by dikes of coarse-grained, 
pink biotite granite. In general the rocks resemble those occurring one 
mile north of Freedhem but apparently little or no hornblende is present 
in the pink granite. 

A group of exposures of considerable extent occurs north and south 
of the section line between sections 7 and 18, T. 41 N., R. 30W., some 
distance east of the quarter post. It is located about 1^ miles south 
of the outcrops in section 6. Two kinds of rock are present, (1) 
a dark gray, fine to medium-grained biotite granite, and (2) a medium 
to coarse-grained pink biotite granite. There is a sharp line of division 
between these two types marked approximately by a diagonal road which 
cuts across the two sections in a northeast-southwest direction. To the 
southeast of the road the outcrops are of pink granite, while to the north- 
west they are of dark gray granite. The pink granite is found also as 
small irregular intrusions in the gray granite. A small quarry occurs 
in the area of the gray granite. The dark gray granite consists of fresh, 
glassy, colorless feldspar and quartz with abundant biotite and some horn- 
blende. Locally hornblende predominates over biotite. The hornblende 
in places is slightly altered to epidote and the biotite to chlorite. The 
pink granite consists mainly of pink feldspar and biotite, but quartz is 
present in small amounts. 

In sections 23 and 24, T. 41 N., R. 31W., about iy 2 miles north 
of the locality known as Gravelville, and about 2 miles southwest of 
the outcrops just described, a large number of granite exposures 
occur in an area of about one square mile. The principal exposures form 
a prominent ridge surrounded on three sides by low marshy areas. Some 
smaller exposures occur within the marshy areas. Several types of gran- 
ite are present, the principal one being a dark gray, medium-grained 
granite with abundant hornblende and considerable biotite. Next to this 
in abundance is a coarse pink biotite granite which seems to be intrusive 
into the hornblende granite, but which since intrusion has suffered de- 
formation. Besides these types a fine-grained black granite occurs which 
contains both hornblende and biotite, the former epidotized, while a 
fourth type consists of fine-grained pink biotite granite, in places porphy- 
ritic. These various types are irregularly intermixed and grade into each 



34 



GEOLOGY OF EAST CENTRAL MINNESOTA 



other. The medium-grained, dark gray hornblende granite is much more 
abundant than the other phases. At one point a small quarry occurs in 
it and here the rock is quite fresh, showing only a slight alteration of the 
ferromagnesian minerals to epidote and chlorite. The pink granites ap- 
pear to be intrusive into it, although the occurrence is very irregular. 
The coarse and fine-grained varieties of pink granite have a similar 
mineral composition and are probably phases of the same rock. They 
contain little or no hornblende. The fine-grained black granite is not 
very abundant. It occurs mainly in the form of irregular included masses 
in the hornblende granite. 

A considerable number of outcrops of silicic igneous rocks occur on 
Skunk River and Hillman Creek, east of Little Falls. The area through 
which the outcrops are scattered is about 15 miles long in a northeast- 
southwest direction, and about 6 miles wide. The town of Pierz is in the 
southwestern part of the area. Most of the outcrops occur on the streams 
or within a short distance from them. As a rule the rocks do not rise far 
above the surface of the soil and in a few localities they have been ex- 
posed only by quarrying. 

The westernmost of these outcrops is at Meyer's quarry south of Fish 
Lake and about a mile west of the station of New Pierz. A very uniform, 
fine-grained, light gray, biotite granite occurs here and some of it has 
been quarried. There are no outcrops other than those in the quarry. 

Several other granite outcrops are found not far from the village of 
Pierz, one occurring on Skunk River near the station of New Pierz, an- 
other one near the junction of Hillman Creek with Skunk River east of 
Pierz, and a third one along the Minneapolis, St. Paul, and Sault Ste. 
Marie railroad, about 2V2 miles east-northeast of the station of New 
Pierz. 

The first of these is an outcrop of medium-coarse, light gray, biotite 
granite, about 50 feet long and 60 feet wide. It is situated about a fifth 
of a mile southeast of the railroad station on the east bank of Skunk 
River. 

The second outcrop is located on the south side of Skunk River about 
a fourth of a mile above the mouth of Hillman Creek. Several exposures 
of gray, banded biotite gneiss are found here. Most of the rock is fine- 
grained and dark, but layers of coarse-grained, light-colored rock occur 
interbanded with the fine-grained rock. Disseminated crystals of garnet 
similar to those occurring in the gneiss at Granite City, farther up Skunk 
River, are abundant. At one point a dike of medium-coarse grained pink 
syenite cuts the gneiss. 

The third one of the outcrops near Pierz occurs in a railroad cut near 
the west side of section 10, T. 40N., R. 30W. The rock is mainly light 



A. GRANITE OUTCROP AT MEYERS QUARRY NEAR PIERZ, MORRISON COUNTY, 
SHOWING HORIZONTAL JOINTING 
PHOTO BY CARL ZAPPFE 




B. INCLUSIONS OF DARK GRAY GRANITE IN PINK GRANITE AT GRAVELVILLE, 

MORRISON COUNTY 



ROCKS OF EAST CENTRAL MINNESOTA 



35 



to dark gray, medium coarse-grained biotite granite. Locally it shows 
a marked gneissoid texture due to the parallel arrangement of biotite 
flakes and their segregation along certain bands. Small dikes of light- 
colored aplite cut the granite. 

Besides these another outcrop of granite is mentioned by Upham 48 
as occurring in the southwestern part of section 22, T. 40N., R. 30W. 
The rock is said to have been quarried. No indications of this outcrop, 
however, were found by one of the writers (Johnston) who visited the 
locality. 

About 8 miles up Skunk River, northeast of Pierz, is a locality 
known as Granite City where a saw mill and village existed previous to 
the Indian outbreak of 1862. Outcrops of coarse-grained gray granite 
and of gray and white banded gneiss occur at this place on both sides 
of the river. The granite contains quartz, biotite, and white feldspar in 
approximately equal amounts. Locally the feldspar predominates, how- 
ever, and occurs in somewhat larger grains, so that it gives the rock a 
porphyritic appearance. In places the biotite shows a tendency to parallel 
arrangement, thus giving gradations into gneiss. Dikes of medium fine- 
grained aplite are found in several places in the granite. 

The gneiss occurs as bands bounded by granite. It consists of inter- 
layered fine and coarse-grained material. The coarse-grained layers con- 
sist mainly of quartz and feldspar and are usually white or light-colored, 
while the fine-grained layers contain abundant biotite and are dark gray. 
At many places the fine-grained layers show fine lamination due to the 
parallel arrangement of biotite. Locally also they contain fine, light- 
colored laminae of fine-grained feldspar, or quartz and feldspar. The 
dark and light-colored layers in the gneiss vary in thickness up to per- 
haps an inch or more, the dark fine-grained layers being thicker and 
making up more of the rock. Red garnet is abundant locally in the gneiss, 
usually occurring in the fine-grained layers as scattered crystals. Horn- 
blende does not occur ordinarily as a constituent either in the granite or 
gneiss, but locally it is found in the latter in large crystals scattered 
through certain layers similarly to the garnet. 

In the NEJ4 of the SW*4 of section 11, T. 41N., R. 29W., about 
3 miles northeast of the Granite City locality just described, a small 
outcrop of granite occurs in the road. The rock consists of fine to coarse- 
grained gray biotite granite similar to that at Granite City, and contains a 
few small lenses of pegmatitic material. No gneiss occurs in this outcrop. 

In the SW% of section 12, T. 41 N., R. 29W., at a locality known as 
Rucker, several small outcrops of rock are found along the west side 



48 Upham, Warren, The geology of Crow Wing and Morrison counties: Geol. and Nat. 
Hist. Survey of Minn., Geol. of Minn., vol. 2, 1882-1885, pp. 580-611. 



36 



GEOLOGY OF EAST CEXTRAL MINNESOTA 



of Skunk River. Most of the exposures consist of a dark lavender or 
greenish gray hornblende rock. It has a porphyritic texture, containing 
phenocrysts of hornblende and gray calcic feldspar, the former predomi- 
nating. The hornblende has a granular appearance rather than the usual 
fibrous texture. The rock is probably a syenite or diorite. 

At this locality one exposure occurring southwest of the bridge over 
Skunk River consists of coarse-grained, dark gray, biotite granite, cut 
by dikes of medium to coarse-grained pink granite. The gray granite 
is fresh in appearance due to the presence of abundant fresh, colorless 
or white feldspar. Biotite is abundant in it and some hornblende occurs. 
The pink granite consists mainly of pink feldspar, but it contains abundant 
quartz and everywhere a little biotite. The feldspar shows very irregular 
crystallization, masses of fine-grained and coarse-grained material being 
closely associated. 

A small knoll consisting of gray and pink granite occurs on the north 
side of Little Skunk River near the east side of section 6. T. 41 N., R. 
28YY., about 2 miles above Rucker. The gray granite is a coarse- 
grained biotite granite, similar in appearance to that at Granite City. It 
contains frequent scattered crystals of hornblende, some of them more 
than half an inch long. The pink granite varies in texture and composi- 
tion. Some phases of it are coarse-grained and almost free from ferro- 
magnesian minerals, while other phases are medium-grained and contain 
abundant scattered biotite. Frequently the coarse and medium-grained 
types are closely associated. A few small dikes of fine-grained, light- 
colored granite are intruded into the gray granite. 

Several other rock outcrops occur on Little Skunk River and on the 
upper portion of Skunk River, both in section 6, T. 41 X.. R. 28W., and 
in section 1. T. 41 X., R. 29W. One of the larger ones of these is found 
near the center of section 1 and consists of granite. Another one is 
mapped by L'pham 49 as occurring along the northern boundary of sec- 
tion 6. The latter, however, could not be found and has probably been 
confused with the outcrop at Rucker as is indicated by the description. 

An interesting group of exposures of various kinds of granite is found 
along Hillman Creek and Little Hillman Creek in an area about 3 miles 
long east and west, and about 1 mile wide. The westernmost of these 
outcrops occur about 4J4 miles east of Pierz in the eastern part of sec- 
tion 7 and western part of section 8, T. 40X., R. 29W. They form an 
elevation along the north side of Hillman Creek and consist of coarse 
gray biotite granite which is intruded by medium-grained pink biotite 
granite. The gray granite locally is porphyritic and contains conspicuous 



19 Upham, Warren, The geology of Crow Wing and Morrison counties: Geol. and Nat. 
Hist. Survey of Minn.. Geol. of Minn., vol. 2, 1 882-1 885, PI. 53 and p. 592. 



ROCKS OF EAST CENTRAL MINNESOTA 



37 



crystals of white feldspar, but for the most part it is even-grained. The 
pink granite shows pegmatitic facies locally, while elsewhere it is in- 
truded as narrow dikes and is fine-grained. 

In the northeastern part of section 8, fine-grained pink and dark gray 
granites are intruded into coarse pinkish gray granite. The Minneapolis, 
St. Paul, and Sault Ste. Marie railroad cuts through two of the outcrops 
and two small quarries occur in them, one to the north and one to the 
south of the railroad. The fine-grained pink granite consists mainly of 
pink feldspar and quartz, but biotite and hornblende occur scattered 
through it. The fine-grained, dark gray granite contains abundant ferro- 
magnesian minerals, both biotite and hornblende. It is not extensive 
areally. The coarse-grained gray granite covers a larger area than the 
other two types. It is fresh and consists of slightly pinkish feldspar and 
quartz with much biotite. The latter is partly altered to chlorite. The 
pink granite intrudes it as dikes and as larger intrusive masses. Pink 
pegmatitic phases occur also. The quarries are both in the pink granite. 

Farther up Hillman Creek, in section 35, T. 41 N., R. 29W., about a 
mile above the locality just described, are the remains of an old logging 
dam. At this place a large number of outcrops occur on both sides of the 
creek and extend for some distance to the north. The Minneapolis, St. 
Paul, and Sault Ste. Marie railroad has cut through one of the larger out- 
crops. Most of the exposures south of the creek consist of coarse-grained 
gray granite with some gneissic phases, while those to the north are 
mainly fine-grained pink granite. The pink granite contains only a small 
amount of ferromagnesian minerals, being composed principally of pink 
feldspar and quartz. It is intruded into the coarse gray granite as dikes 
and large masses. In the railroad cut a nearly horizontal sill of pink 
granite varying up to two feet in thickness, and bounded above and below 
by coarse gray granite, occurs in both walls of the cut almost along the 
entire length. North of Hillman Creek, the intrusive masses of pink 
granite are much larger, though the texture is uniformly fine-grained. 
The coarse gray granite is a biotite granite similar to that already de- 
scribed as occurring to the southwest. Portions of it apparently have 
suffered considerable deformation and recrystallization with the result 
that masses of coarse banded gneiss have developed. These, however, 
are of restricted occurrence. Not all the dikes are of pink granite. There 
are also dikes of fine-grained gray biotite granite which cut the coarse- 
grained granite. One of these is exposed in the railroad cut. 

On Little Hillman Creek in section 9, T. 40N., R. 29W., about a mile 
east of the outcrops in sections 7 and 8, there is another group of ex- 
posures of gray and pink granite. The principal rock is a coarse-grained 
biotite granite, in general having a gray appearance, but in places con- 



38 



GEOLOGY OF EAST CENTRAL MINNESOTA 



taining pink feldspar in abundance. This rock is intruded by dikes of 
fine-grained light gray and dark gray granite and fine-grained white to 
pink aplite. The light gray granite contains but a small amount of ferro- 
magnesian minerals, while the dark granite contains abundant biotite and 
some hornblende. The aplite dikes are in places made up almost entirely 
of quartz and pink feldspar. Locally, however, scattered ferromagnesian 
minerals are present. The gray coarse granite has suffered deformation 
and in places shows gneissic structure. 

A few small outcrops consisting mainly of very coarse-grained pink 
biotite granite are found near the west side of section 18, T. 39N., R. 
30W., about three miles southwest of Buckman. They occur in a barn- 
yard a short distance from the north-south township road. The rock is 
considerably weathered and friable. It contains inclusions of a fine- 
grained dark gray rock. One of the outcrops consists of fresh pink 
granite with texture varying from fine-grained to coarse-grained in dif- 
ferent parts. 

STEARNS, BENTON, SHERBURNE, AND WESTERN MILLE LACS COUNTIES 

The region surrounding Saint Cloud and Sauk Rapids contains a 
large number of rock outcrops, principally granite and subordinately 
diorite and gabbro. They are situated mainly in the eastern part 
of Stearns County and in the western parts of Benton and Sherburne 
counties, and extend approximately from Watab on the north to Linden 
township on the south, and to Rockville on the southwest. The area in 
which outcrops occur is about 16 miles long north and south, and about 
12 miles wide. Outside of this area, however, there are a few scattered 
outcrops, such as the exposures of granite and of Cretaceous sediments 
along Sauk River near Coldspring and Torah (Richmond), Stearns 
County, and the granite exposures on the upper Elk River about 8 miles 
northwest of Foley, Benton County. 

Most of the outcrops occur in groups, but isolated, scattered outcrops 
are also abundant. The most northerly of the groups of outcrops is that 
south of Watab on the east side of Mississippi River. Three rocky 
knolls are found here between the tracks of the Northern Pacific Rail- 
way and the river. The knolls consist principally of a dark, fine-grained 
hornblende rock, probably a diorite. Locally between the knolls fine to 
medium-grained pink granite which appears to be intrusive in the diorite 
crops out. In the knoll farthest east near the railroad a quarry occurs 
in the pink granite, in which several phases are shown. 

From Watab southward to Sauk Rapids exposures of diorite and pink 
and gray granite occur at frequent intervals. Most of them are found 
from 1 to 3 miles east of Mississippi River, but west of the river 
there are also scattered outcrops. About 1V2 miles north of Sauk Rapids 



ROCKS OF EAST CENTRAL MINNESOTA 



39 



and 2 miles east of the river, are situated a number of quarries. Several 
different types of granite are taken from these, including medium coarse- 
grained pink hornblende granite and light to dark gray biotite and horn- 
blende granite. 

At Sauk Rapids a number of outcrops are found. On the east side of 
Mississippi River are exposures of dark gray hornblende granite, while 
on the west side below the bridge is an exposure of coarsely porphyritic 
rhyolite porphyry containing both feldspar and quartz phenocrysts in a 
gray, finely crystalline matrix. 

The largest group of outcrops in the Saint Cloud region is situated 
about 2 miles southwest of Saint Cloud and occupies an area about 4 miles 
long north and south and about 3^2 miles wide. 50 Pink and red granite 
and syenite exposures are abundant, being scattered at short intervals 
over the entire area, some of them being several acres in extent. The 
rock is described by Upham as principally syenite with predominating 
hornblende but also containing abundant mica. 

Near Rockville, about 6 miles southwest of this group of outcrops, a 
number of massive 51 exposures of coarse-grained gray, biotite granite 
occur. Several of these exposures are forty or fifty rods in extent and 
rise forty or fifty feet above the surrounding level. They are found along 
Mill Creek south of the village and along Sauk River north of the village. 

In Saint Augusta township about 4 miles east-southeast of Rock- 
ville, 52 an exposure of pink biotite granite is reported to occur near the 
center of section 19, T. 123N., R. 28W., while an outcrop of gray granite 
is reported in the northwestern corner of Linden township about 2^ 
miles south of Saint Augusta. 53 

The outcrops at Coldspring and Torah (Richmond) in Wakefield 
township are described as consisting mainly of coarse reddish syenite. 
The Coldspring outcrops occupy an area a quarter of a mile square. 
Several exposures of very hard, dark diorite are reported as occuring 2 
miles east of Torah (Richmond). These are the southernmost outcrops 
of igneous rock in east central Minnesota. 

In the northwestern part of Sherburne County several outcrops occur 
in Haven township 54 southeast of East Saint Cloud. A number of 
quarries are found near the State Reformatory about 2 miles from East 
Saint Cloud in a rock said to be fine-grained gray to red syenite. Ex- 



*> Upham, Warren, The geology of Stearns County: Geol. and Nat. Hist. Survey of Minn., 
Geol. of Minn., vol. 2, 1882-1885, pp. 455-458. 

Bowles, O., and Grout, F. F., Unpublished map. 
n Upham, Warren, op. cit., p. 454. 

62 Upham, W., op. cit., p. 455. 

63 Bowles, O., and Grout, F. F., op. cit. 

64 Upham, Warren, The geology of Benton and Sherburne counties: Geol. and Nat. Hist. 
Survey of Minn., Geol. of Minn., vol. 2, 1882-1885, pp. 426-444. 



40 



GEOLOGY OF EAST CENTRAL MINNESOTA 



posures occur over a considerable area. About 2 miles south of the Re- 
formatory in the west central part of the township is an outcrop of coarse 
whitish gray syenite 55 covering 4 or 5 acres and rising as a bare rounded 
knoll above the surrounding prairie. 

The outcrops northeast of Foley near the center of Benton County 
are reported by Upham to be only a few rods in extent and to consist of 
reddish syenite. 

A group of granite outcrops of considerable extent is found northeast 
of the Saint Cloud region along the upper courses of the West Branch 
of Rum River and Saint Francis River in northeastern Benton and 
western Mille Lacs counties. 56 The outcrops on the West Branch of 
Rum River both in Benton and Mille Lacs counties are said to consist 
of remarkably uniform, coarse-grained, reddish syenite. They occur 
along the river through an extent of about 2 or 3 miles. Many different 
exposures are found, most of them small ledges occurring along the 
banks or in the channel of the stream. The westernmost group of ex- 
posures is a short distance above the mouth of Stony Brook in Benton 
County, while the easternmost exposures occur about 3 miles west of 
Milaca. 

The outcrop near the headwaters of Saint Francis River is about four 
miles west of the westernmost outcrop on the West Branch of Rum River 
in the northern part of section 20, T. 38X., R. 28W. It is reported to be 
about 25 rods long by 15 rods wide and to consist of reddish syenite cut 
by an east and west trap dike. The dike is from a foot to a foot and a 
half wide and has fine-grained syenite on the north side and coarse- 
grained syenite on the south side. 

NORTHERN MILLE LACS, KANABEC, SOUTHERN AITKIN, AND WESTERN PINE COUNTIES 

A number of outcrops of several different kinds of silicic igneous rock 
occur south and southwest of Onamia in the northern part of Mille Lacs 
County, some being on Rum River and some on Bradbury Brook, a 
tributary of Rum River. 

The most northerly outcrop on Rum River is located about 5 miles 
south of Onamia near the ruins of an old logging dam. There is a bridge 
over the river at this point, and the exposures are on the east side of the 
river both north and south of the bridge. The principal rock is a light- 
colored, medium to coarse-grained granite, much of which has developed 
a gneissic structure and contorted, obscure banding. The banding is due 
in part to the arrangement of biotite in streaks and in part to a difference 
in size of grains of the different bands. The coarse granite is cut by 
several varieties of dark-colored dikes. One is a very dark, fine-grained 

65 Upham, W., op. cit., p. 432. 

54 I'pham, Warren, The geology of Mille Lacs and Kanabec counties: op. cit., pp. 612-628. 



A. GRANITE EXPOSURES ON RUM RIVER, SOUTH OF ONAMIA, MILLE LACS COUNTY 




B. GRANITE EXPOSURES ALONG HAY CREEK, A BRANCH OF SNAKE RIVER, 
KANABEC COUNTY 



ROCKS OF EAST CENTRAL MINNESOTA 



41 



biotite granite while the other is a dark, dense rock gray to greenish and 
fine-grained. The latter rock contains abundant pyrite. 

The other outcrops on Rum River occur in section 7, T. 40N., R. 26W., 
and section 12, T. 40N., R. 27W. They form a series of three outcrops, 
two occurring in the NWJ4 of NW34 of section 7 and one in the NE^4 
of NEJ4 of section 12. All are found within a distance of about a half 
a mile along the stream. 

The first of these outcrops, going down stream, consists of dark gray, 
fine-grained biotite granite, which has been squeezed so that the biotite 
has developed a parallel arrangement without segregation into bands thus 
giving the rock a schistose appearance. The rock, however, is quite 
massive. 

The second outcrop consists of dense, fine-grained, mica schist. The 
rock has a dark greenish gray color and contains abundant flakes of 
muscovite, as well as biotite. It appears to be similar to the rock exposed 
in the outcrop last mentioned, but has suffered further deformation. 
Dikes of a dark-colored fine-grained feldspathic rock are found in it. 

The third outcrop consists of light-colored medium coarse-grained 
granite consisting mainly of quartz and feldspar, but containing scattered 
biotite. 

The exposures along Bradbury Brook consist mainly of light-colored 
gray or brownish granite with local gneissic phases. Three of the ex- 
posures occur on or near the South Fork of Bradbury Brook in section 
25, T. 41N., R. 27W., and section 30, T. 41 N., R. 26W., and another one 
occurs in section 13, T. 41 N., R. 27W., a short distance east of the North 
Fork of Bradbury Brook. 

The outcrop in section 13 is located on the quarter line about 1,000 * 
feet west of the north-south township road between T. 41 N., R. 27W., 
and T. 41 N., R. 26YY. The principal rock is a gray medium-grained 
granite with abundant biotite, which is more or less concentrated along 
parallel streaks due to deformation and recrystallization and gives the 
rock a gneissic appearance. It does not show distinct banding, however. 
The biotite granite is cut by several dikes of a very light-colored fine- 
grained to medium-grained quartz-feldspar rock (aplite) containing a 
little scattered biotite. 

The northernmost of the exposures near the South Fork of Bradbury 
Brook occur along the township road between section 25, T. 41 N., R. 
27W., and section 30, T. 41 N., R. 26W. about 1,400 feet south of the 
section corner. The rock here consists of medium coarse-grained light- 
colored granite with abundant quartz and feldspar and scattered biotite. 
Locally it shows a tendency to a gneissic texture, which is not very 
marked. This rock contains inclusions of a fine-grained, dark granite 
with abundant ferromagnesian minerals. 



42 



GEOLOGY OF EAST CENTRAL MINNESOTA 



About 400 feet south of these exposures at the bridge over the South 
Fork of Bradbury Brook, a small outcrop of fine-grained, gneissoid gran- 
ite occurs east of the road. The biotite shows in conspicuous parallel 
lines. 

About 300 feet west of the outcrop at the bridge just mentioned, there 
is a dam on the South Fork of Bradbury Brook near which are found 
several exposures of medium-grained gray biotite granite. Some of the 
rock is light-colored and consists mainly of quartz and feldspar with scat- 
tered biotite, but in some of it the ferromagnesian minerals equal in 
amount or predominate over the quartz and feldspar and such phases are 
dark. 

Upham 57 mentions the occurrence of several other rock exposures 
along Rum River and Bradbury Brook. Those on Rum River are said 
to be several miles above the mouth of Bradbury Brook and to consist 
of syenite, hornblende rock, gneiss, granite, and greenstone. Outcrops 
of this description have been looked for but have not been found by the 
writers. The outcrop on Bradbury Brook is reported to be on the South 
Fork 3 or 4 miles above its junction with the North Fork. As these ex- 
posures were not visited by Upham but were reported on the authority 
of others, it is probable that they have been confused with certain of the 
outcrops described above. 

In the NWJ4 of SE34 of section 27, T. 42N., R. 26W., about 3 miles 
northeast of Onamia, exposures of granite and diorite are reported by 
Grout 58 to occur in the bottom of a cut along the Minneapolis, St. Paul, 
and Sault Ste. Marie Railroad. The outcrop rises from 2 to 3 feet above 
the grade and extends for 185 feet along the north side. Only a few 
exposures are found on the south side of the cut. The rock consists of 
alternate masses of fine-grained light gray biotite granite and dark gray 
medium coarse-grained diorite. The contacts are sharp and no change in 
the size of the grain is noticed on approaching them. Along one contact 
is a small pegmatitic band grading into the granite. A few small dark 
inclusions or blotches of ferromagnesian minerals are found in the 
granite. 

About 5 miles southeast of Wahkon, Mille Lacs County, in section 
3, T. 41 N., R. 25W., a quarry has recently been opened in light gray 
granite. It is located in the northeast corner of the section in a bend of 
Knife River. The granite is of two kinds, one a medium coarse-grained 
rock, and the other finer grained. Both are very light colored and con- 
tain only a small percentage of ferromagnesian minerals. Their relation 
is such as to indicate that the fine-grained granite is intrusive into the 



K Upham, W., op. ext., p. 617. 

88 Grout, F. F., Oral communication. 



ROCKS OF EAST CENTRAL MINNESOTA 



43 



coarse-grained granite. The fine-grained rock resembles very closely 
that found at Meyers quarry west of Pierz and is hard and fresh. Dikes 
and irregular masses of fine-grained aplite cut both varieties of granite. 

A number of granite outcrops are found in the region of Ann Lake, 
4 to 7 miles northwest of Mora, Kanabec County. Near the boundary 
between sections 19 and 20, T. 40N., R. 24W., just north of the east and 
west road which runs along the south line of these sections, several ex- 
posures of medium-grained, pinkish gray biotite granite occur. They 
probably continue south of the road into sections 29 and 30. The prin- 
cipal exposure forms a small hummock overlooking a marsh a short dis- 
tance north of the road. The rock here is a light pink biotite granite 
with abundant quartz and both white and pink feldspar. 

Upham 59 reports the occurrence of ledges of gray fine-grained granite 
along Ann River in sections 30, 29, and 32 half a mile to a mile south 
of the locality mentioned above. He states that similar outcrops are re- 
ported as occurring also along Little Ann River north of Ann Lake in 
sections 26 and 14, T. 40 N., R. 25W. 

Quarries in granite are located at Warman in the southeast corner 
of section 6 and the southwest corner of section 5, T. 41 N., R. 23W. 
The granite is very light gray, medium-grained, with abundant quartz 
and white feldspar, and considerable amounts of scattered biotite. The 
rock is very even-grained and shows no abrupt changes in texture or 
mineral composition. Most of it is fresh and shows little or no altera- 
tion of the feldspars. At one of the quarries there are a few dikes of 
white feldspar-quartz pegmatite, containing scattered large thin biotite 
flakes. 

Numerous outcrops of sedimentary and igneous rocks are found along 
Snake River in the northern part of Kanabec County. The southern- 
most of these exposures, consisting of several outcrops of flat-lying pink 
and red sandstone, are found in the northwestern part of section 23, T. 
42N., R. 23W. The rock probably forms a part of the supposed upper 
Keweenawan sandstone which extends southwestward from the western 
end of Lake Superior. The outcrops are described later. 

The next group of exposures to the north up Snake River is found 
beyond the big bend in the river in sections 15 and 16 of the same town- 
ship. The river at this point flows eastward and the locality, which is 
known as Lower Falls, is a short distance west of the new iron bridge 
over the river. The exposures consist of gray and pink biotite granite 
with large included masses of biotite schist. 



69 Upham, Warren, The geology of Mille Lacs and Kanabec counties: Geol. and Nat. Hist. 
Survey of Minn., Geol. of Minn., vol. 2, 1882- 1885, pp. 617 and 618. 



44 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Up the river beyond this group of exposures there is a number of 
scattered granite outcrops as far as the mouth of Hay Creek, while north 
and northwest of the mouth of Hay Creek exposures again increase in 
abundance and cover an area of considerable size in sections 16 and 9. 
In this area many large outcrops occur both westward along Hay Creek 
and northward along Snake River. They extend up Hay Creek for 
nearly a mile and up Snake River from the mouth of Hay Creek about a 
third of a mile to a place known as Upper Falls where a reef of rocks 
runs across the stream. The rocks in this area are mostly gray and pink 
biotite granite, but locally there are pegmatitic fades and also included 
bands of biotite schist. There are also several fine-grained basic dikes. 

Above Upper Falls only a few scattered outcrops are found along 
the river as far as the mouth of Cowan Brook. About 800 feet above 
the mouth of Cowan Brook, however, near the northern boundary of 
section 9, a number of small exposures of schist and granite occur on 
both sides of Snake River, while about a third of a mile farther up an 
important group of exposures is found along Snake River and east in 
the area between Snake River and Cowan Brook. These outcrops are 
in the southeastern part of section 4. They consist mostly of light- 
colored granite with local gneiss and pegmatite. The entire area occupied 
by the outcrops of Lower Falls, Hay Creek, Upper Falls, and Cowan 
Brook is about 2 miles wide east and west and 2 l / 2 miles long north and 
south. 

The principal exposures at Lower Falls extend along the river for 
a distance of about 3,000 feet, but small scattered outcrops continue 
both up and down the river. By far the most important rock is 
gray and pink biotite granite. This rock is medium-grained and hard 
and fresh, containing abundant quartz. There seems to be no distinct 
difference in age between the gray and the pink types, the former grading 
into the latter with increasing pink feldspar. As far as can be deter- 
mined from these exposures, there is no sharp line between them. The 
amount of ferromagnesian minerals in the granite is usually small, giving 
the rock a light color. 

The schist, which occurs as large masses surrounded by granite, is 
principally a fine-grained, quartzose biotite schist, but some of it is 
coarsely crystalline. Locally muscovite occurs with the biotite and along 
certain streaks and bands much hornblende is developed. Garnet also is 
commonly disseminated through the rock, while certain hard quartzitic 
lenses have an interesting development of molybdenite around the bor- 
ders. The larger of the schist masses have a fairly uniform strike east 
and west and dip to the south at angles varying between 50 and 70 . 
They are cut by dikes of pink granite and also locally by pink pegmatite 



PLATE V 




A. GRANITE AND MICA SCHIST OUTCROPS AT LOWER FALLS OX SNAKE RIVER, 

KANABEC COUNTY 




B. GRANITE EXPOSURES AT UPPER FALLS ON SNAKE RIVER, KANABEC COUNTY 



ROCKS OF EAST CENTRAL MINNESOTA 



45 



dikes consisting of quartz and pink feldspar with abundant muscovite. 
The association shows that the granite is intrusive into the schist, the 
masses of the latter being fragments caught in the granite. If the schist 
is of upper Huronian age, as is generally supposed, then the granite is 
post-upper Huronian and may be equivalent to certain other Keweenawan 
granitic rocks of the Lake Superior region. Because of the isolated 
situation of these rocks, however, correlation is difficult, and the age 
relationships are therefore uncertain. 

The rock exposures along Hay Creek and along Snake River near 
Upper Falls, are in general similar to those at Lower Falls in that they 
consist largely of medium-grained gray and pink granite. There is more 
pegmatitic rock, however, in these exposures and locally also the granite 
is quite fine-grained in texture. Pegmatite is especially abundant at 
Upper Falls and along both sides of the river below Upper 4 Falls. In 
places, large masses of very coarsely crystalline material occur, consisting 
of pink feldspar, quartz, and muscovite, with locally some biotite. Usually 
the muscovite is in large flakes, but some of the pegmatite has suffered 
deformation and the muscovite is recrystallized into small flakes having 
a fan-shaped, foliated structure around fragments of quartz and feld- 
spar. The granite also in a few places has suffered deformation and has 
developed a gneissoid texture. In one or two places it has even become 
coarsely schistose. Dikes of very fine-grained, dark greenish gray sub- 
silicic igneous rock occur in the river at Upper Falls and at the dam on 
Hay Creek about a mile above its mouth. The one at Upper Falls shows 
diabasic texture while the one on Hay Creek appears to be a diorite. 
Masses of fine-grained quartzose biotite schist similar to that at Lower 
Falls occur locally in the Hay Creek and Upper Falls outcrops, but their 
development is not as abundant as in the former locality. 

The exposures along Snake River between Upper Falls and the mouth 
of Cowan Brook as well as those a short distance above the mouth of 
Cowan Brook consist of light-colored pink and gray biotite granite with 
a considerable quantity of fine-grained biotite schist. Those above the 
mouth of Cowan Brook contain a large pegmatite dike. 

The outcrops situated in the area between Snake River and Cowan 
Brook, about a third of a mile above the mouth of the latter, consist of 
light-colored biotite granite. Both the pink and gray types occur, most 
of the rock being very low in ferromagnesian minerals. Muscovite in 
small flakes is abundantly disseminated through it locally. In a few 
places there are gneissoid phases, showing distinct banding. 

In the southwestern part of section 21, T. 43N., R. 23W., Aitkin 
County, about 3 miles north of the Cowan Brook exposures, there are 
several outcrops of schistose and banded gneiss along Snake River. The 



46 



GEOLOGY OF EAST CENTRAL MINNESOTA 



locality is known as Malloy (or Malloid) Dam. The rock seems to be 
a deformed and recrystallized granite. It varies from pink or white to 
dark gray in color. Most of it is finely crystalline biotite gneiss, biotite, 
quartz, and feldspar being arranged along fine laminae. The laminae of 
biotite are very thin, while those of quartz and feldspar are thicker and 
more irregular. There may be 20 or 30 thin laminae of biotite to the 
inch separated by thicker laminae of quartz and feldspar. Usually the 
successive laminae are closely similar, giving the rock a homogeneous 
and schistose appearance. Locally, however, there is a distinct banding 
of light and dark layers and the rock becomes a banded gneiss. The dip 
of the scbistosity and banding varies from 65° to 8o c north, the strike 
being nearly east and west. At one point a dike of pink granite cuts 
across the gneiss. It shows no deformation whatever. This dike rock 
is identical in appearance with the pink granite occurring along the Snake 
River in Kanabec County which contains included masses of schist. 
It appears that the gneiss and schist are older formations which have 
been rendered schistose previous to the intrusion of the granite. The 
granite predominates to the south in Kanabec County, while gneiss and 
schist predominate to the north in Aitkin, Pine, and Carlton counties. 

Southwest of McGrath, a station on the Minneapolis, St. Paul, and 
Sault Ste. Marie Railroad east of Mille Lacs Lake, many outcrops of 
fairly coarse pink augen-gneiss occur on both sides of the east-west road 
running between section 1 and section 12, and section 2 and section II, 
T. 43N., R. 24W. The outcrops extend for a mile along the road and all 
seem to be composed of the same kind of rock. The rock at one time 
was apparently a fairly coarse-grained granite with numerous prominent 
crystals of feldspar and some of quartz. Deformation occurred, and 
during this process the biotite and some of the quartz and feldspar were 
crushed and recrystallized. Other quartz and feldspar crystals, however, 
resisted deformation and remained as lentils or buttons around which 
the crushed material recrystallized. giving the rock a wavy, foliated tex- 
ture. The buttons of quartz and feldspar are coarse-grained, many con- 
sisting of a single crystal, while the crushed and recrystallized material 
is fine-grained. The foliated structure is fairly regular. It strikes about 
X. 70" \Y. and the dip is vertical or steeply inclined to the south. 

Augen-gneiss similar to that occurring near McGrath, as well as 
banded and schistose gneiss, are found cropping out at several places 
west and south of Denham, a station on the Minneapolis. St. Paul, and 
Sault Ste. Marie Railroad in Pine County, about 20 miles northeast of 
McGrath. The distribution of the outcrops indicates that the southern 
part of Aitkin County and the northwestern part of Pine County are 
largely underlain by gneiss, which is bounded on the south by granite 



ROCKS OF EAST CENTRAL MINNESOTA 



47 



and on the north by metamorphosed sediments such as schist and slate. 

The most westerly of these exposures are in the southwestern part 
of section 35, T. 45N., R. 22W., Aitkin County, along the north and 
south road south of Arthyde. Two exposures occur in the road. One 
of them consists of pink augen-gneiss very similar to that at McGrath, 
while the other consists of finely laminated, dark-colored schistose gneiss, 
with thin lentils of crushed pink feldspar and quartz. The dark color 
is due to the abundance of biotite which occurs in fine laminae. The 
rock is probably a more metamorphosed phase of the augen-gneiss. 

About 2 miles west of Denham a rock cut along the railroad in sec- 
tion 21 T. 45N., R. 21W., Pine County, shows dark-colored, schistose 
gneiss. The degree of metamorphism varies in different parts so that in 
places the rock is a perfect schist, finely laminated, consisting largely of 
biotite, while elsewhere it is more massive and contains abundant feld- 
spar and quartz. This variation produces a rough banding more or less 
parallel to the foliation. The foliation or schistosity strikes roughly east 
and west, and dips to the north at angles varying between 70 and 8o°. 
The rock is similar to that in one of the outcrops south of Arthyde but 
the "augen" structure is less marked. 

About 1% miles south of the locality just described, on the north 
and south road between sections 33 and 34, T. 45N., R. 21W., pink 
augen-gneiss similar to that at McGrath, outcrops at several places while 
a mile further south between sections 3 and 4, T. 44N., R. 21W., other 
exposures of the same type of rock are found. In the latter locality the 
gneiss shows a considerable amount of muscovite. 

Several exposures of gneiss with associated hornblende schist are 
located in the western part of the township of Bremen, T. 44N., R. 21W. 
The most northerly of these are found along the northern headwaters of 
Pine River in the northeastern corner of section 17 and adjacent parts 
of sections 16, 8, and 9. An outcrop on the west bank of the stream 
near the east quarter post of section 17 consists of pink gneiss, very 
coarse and containing muscovite and biotite along foliation planes. The 
foliation strikes approximately east and west and dips about 50 S. 
Along the north and south section road, near the northeast corner of sec- 
tion 17, and extending into the adjoining sections to the east and north, 
are several exposures of dark reddish gneiss with closely spaced foliation 
planes and of very dark, almost black, finely laminated hornblende schist. 
The gneiss contains abundant mica along foliation planes, the close spac- 
ing of which indicates intense deformation and metamorphism. The 
hornblende schist shows continuous fine laminae of hornblende alter- 
nating with laminae of quartz and feldspar. The hornblende is by far 
the most abundant mineral in the schist and produces a schistosity parallel 



48 



GEOLOGY OF EAST CENTRAL MINNESOTA 



to the lamination. The hornblende schist appears to be a phase of the 
gneiss but the relation of the two rocks could not be determined. The 
general dip of the foliation in these outcrops is 35 S. 

The other outcrops in the township of Bremen are located along a 
branch of Pine River on both sides of the north and south section line 
between sections 19 and 20. The principal exposures are found on both 
the north and south banks of the stream south of the east and west 
quarter line. They consist of pink, fresh looking, distinctly foliated 
gneiss, some of which shows a somewhat indistinct "augen" texture and 
some the more regular interbanding of feldspar and quartz with mica. 
The former as a rule is more coarsely crystalline than the latter. The 
ferromagnesian minerals occur along foliation planes, both biotite and 
hornblende being present. The foliation planes as a rule are thin, irregu- 
lar, and discontinuous, but locally they widen forming lense-like bunches 
of dark minerals. The general dip of the foliation is to the south, vary- 
ing between 40 and 8o°. 

A group of small rock exposures occurs a short distance south of the 
stream near the south line of section 20 not far from the southwest cor- 
ner. These are of gneiss. Hornblende and biotite schists also occur 
here. The gneiss varies in texture from very coarsely crystalline to 
medium finely crystalline and in color from gray to pink. Nearly all of 
it is distinctly but irregularly banded. In some places the segregation of 
light and dark minerals is very perfect, so that white or pink bands of 
intermixed feldspar and quartz ranging up to half an inch thick are 
interlayered with darker bands of feldspar, quartz, and ferromagnesian 
minerals. The hornblende and biotite schists are dark -colored, often 
black or greenish black and generally are finely and regularly laminated 
due to interlayering of fine-grained feldspar, quartz, and ferromagnesian 
minerals, mainly hornblende. The ferromagnesian minerals are much 
more abundant than the light-colored minerals, but locally layers up to a 
fourth of an inch in thickness, consisting almost entirely of feldspar and 
quartz, occur interlaminated with the dark minerals. Some of the schist 
shows irregular undulatory lamination and marked schistosity, usually 
being more coarsely crystalline and appearing to have suffered greater 
metamorphism than the more regularly banded schist. Most of the schist 
is fairly pure hornblende schist, but local phases show a considerable ad- 
mixture of biotite. Interesting phases, gradational in texture as well as in 
mineral composition, occur between the gneiss and the schists. In some 
of these the light and dark-colored minerals are almost equally abundant 
and light-colored layers are interbanded with dark-colored layers. Some 
of them show gneissoid, and some show schistose textures. It seems 
quite clear, therefore, that the gneiss and schists belong to the same rock 



ROCKS OF EAST CENTRAL MINNESOTA 



40 



mass, the one representing the more silicic and the other the less silicic 
portions of it. The latter appear to have yielded more readily to de- 
formational forces. The dip of the foliation and schistosity is to the 
south at angles varying between 40 and 70 . 

A number of exposures of augen-gneiss of considerable size occur in 
the southeastern part of the NE^4 of section 36, T. 45N., R. 21W. The 
outcrops are about 1^2 miles south of Denham and a short distance south 
of a group of exposures of interesting metamorphosed sedimentary rocks 
found in the southeastern portion of section 25 which is described later. 
An isolated exposure of dark greenish black hornblende-biotite schist is 
found north of the gneiss outcrops about half-way between them and 
the outcrops of metamorphosed sediments. 

Most of the rock composing the gneiss exposures is a gray to pink 
augen-gneiss with distinct undulatory foliation. The large crystals form- 
ing the augen are of pink feldspar and present all stages of crushing, the 
foliation in the ground-mass curving around them. The ground-mass is 
mainly quartz, feldspar, biotite, and muscovite, all finely crystalline. In 
some phases muscovite is present almost to the exclusion of biotite. A 
small mass of biotite schist is found in one of the outcrops. It contains 
considerable feldspar and is probably a sheared basic inclusion. 

The hornblende-biotite schist found north of the gneiss outcrops is 
similar to that occurring in Bremen township. It shows a distinct, ir- 
regular schistosity, but the lamination is not very marked. Hornblende 
predominates,, while biotite and quartz and feldspar are present in minor 
amounts. This is the easternmost outcrop of gneiss known in central 
Minnesota. 

OUTCROPS OF SUBSILICIC IGNEOUS ROCKS 
TODD COUNTY 

Outcrops of several kinds of coarse-grained, subsilicic igneous rocks 
occur in the northern part of Todd County. They are situated on Long 
Prairie River at the mouth of Fishtrap Creek and along Fishtrap Creek 
for about 1,300 feet above its mouth. The principal exposures consist 
of coarse-grained, dark greenish or brownish black gabbro. The largest 
outcrop of this rock is at the mouth of Fishtrap Creek, but numerous 
small outcrops extend southward up the creek, occurring on both banks 
of the stream. The last exposure up stream is an isolated outcrop found 
on the west bank a short distance above the first road bridge. This bridge 
is 1,000 feet from the mouth of the creek. 

Besides the gabbro there are a few outcrops of anorthosite. One 
of these is on the east side of Fishtrap Creek near its mouth, and another 
is at the road bridge over Fishtrap Creek mentioned above. 



5" 



GEOLOGY OF EAST CENTRAL MINNESOTA 



The gabbro has suffered considerable alteration so that it consists 
principally of feldspar and secondary hornblende. The feldspar is more 
or less altered to saussurite and locally to muscovite and quartz. Apatite 
and magnetite are abundant in the rock and biotite also is present. The 
hornblende is secondary after diallage, traces of which are found in a 
few of the outcrops, notably in the one above the bridge. The rock in 
its original fresh state, therefore, consisted mainly of feldspar and dial- 
lage with biotite, apatite, and magnetite as accessory constituents. The 
anorthosite is composed almost entirely of feldspar, some of which has 
been partly altered to saussurite and some to muscovite and quartz. 

A short distance east of the station of Philbrook, about 2,000 feet up 
Fishtrap Creek from the southernmost of the exposures just mentioned, 
there are a few small outcrops of coarse-grained greenish rock very much 
altered. Most of the rock is composed of saussuritized feldspar, green 
hornblende, and epidote, but some phases of it consist almost entirely of 
altered feldspar. Magnetite is abundant. The rock, although somewhat 
lighter in color, is similar in texture to the gabbro and anorthosite occur- 
ring near the mouth of Fishtrap Creek and it seems probable that it 
represents a further alteration phase of these rocks. 

MORRISON', BENTON, AND STEARNS COUNTIES 

Several exposures of subsilicic and intermediate igneous rock are 
found in the vicinity of Little Falls, Morrison County. One of these 
is at Williams' quarry in the northeast quarter of section 13, T. 129N., 
R. 30W., about a mile northeast of Little Falls. The rock varies greatly 
in texture at different points in the quarry. Some of it is dense and fine- 
grained and has a uniform dark gray color, while other phases are 
coarser grained and have a speckled appearance due to intermixed light 
and dark-colored minerals. The latter vary considerably in size of grain, 
as well as in color. In some, feldspars predominate ; in others, ferro- 
magnesian minerals. The various rocks have a fairly uniform mineral 
composition, except that the coarser grained varieties have suffered 
greater alteration. 

The fine-grained type consists of feldspar and pyroxene, the former 
predominating. It is fairly even-grained, but occasional larger crystals 
of pyroxene occur. Fine particles of magnetite are scattered throughout 
the rock. Much of the feldspar shows partial alteration to granular 
kaolin, while the pyroxene shows practically no alteration. The coarser 
grained type consists mainly of feldspar and hornblende, the latter being 
secondary after pyroxene. Locally much of the hornblende shows ir- 
regular cores of pyroxene, the hornblende forming a border of varying 
thickness. Most of the hornblende shows pleochroism from bluish green 
to light yellowish green. Some of it is greenish brown. Both varieties 



ROCKS OF EAST CENTRAL MINNESOTA 



5i 



of hornblende show pyroxene cores in places, but elsewhere the pyroxene 
has entirely disappeared. The feldspar is slightly altered to kaolin. A 
small amount of biotite (or phlogopite) is quite generally distributed 
through both types of rock, while locally large, very thin, flakes of phlogo- 
pite occur, giving the rock a bronzy appearance. In the phases of the 
rock rich in phlogopite, the pyroxene is quite fresh, and much brown 
hornblende occurs in irregular masses which has the appearance of being 
original. It is quite probable that a considerable amount of both brown 
and green hornblende is original. Locally the feldspar predominates 
over the ferromagnesian minerals to such an extent as to give the rock 
a very light color. Specks and irregular grains of magnetite are numer- 
ous, especially in association with pyroxene, while grains of quartz occur 
rarely. In general the rock has the composition of a diorite. 

The diorite at Williams' quarry and the syenite occurring on the east 
bank of Mississippi River at Little Falls, are within a region in which 
slate and schist of sedimentary origin predominate. As the metamor- 
phosed sediments have a fairly definite and regular structure throughout, 
it would seem that the igneous rocks are probably intrusive into them, 
although contacts between them have not been found. 

An outcrop of hard, dark rock occurs on the west bank of Mississippi 
River about three quarters of a mile below the bridge of the Minneapolis, 
St. Paul, and Sault Ste. Marie Railroad, in section 32, T. 128N., R. 29W. 
It forms rapids in the river, known as Blanchard's Rapids, and out- 
crops of it are found on the east bank also. The rock is medium coarse 
grained and contains abundant pyroxene and calcic feldspar with locally 
phlogopite. It resembles gabbro in composition. Only a short distance 
above these outcrops, along the river bank, are exposures of staurolitic 
and garnetiferous biotite schist which are apparently the southernmost 
outcrops of these rocks on Mississippi River. The relation of the schist to 
the igneous rock could not be definitely determined. Judging from the fact, 
however, that the cleavage of the schist strikes in the direction of the 
gabbro it would seem that the latter is intrusive, as is the case with the 
other subsilicic or dark igneous rocks in the vicinity of Little Falls. 

The occurrence of a dark-colored hornblende rock on Skunk River 
near the locality known as Rucker, in the northwestern part of Morri- 
son County, has already been mentioned. The rock is dark grayish in 
color and contains abundant specks of granular hornblende. It is prob- 
ably a hornblende syenite, but may approach a diorite in composition. 

The outcrops of various rocks of intermediate composition in Benton 
and Stearns counties have been mentioned in connection with the descrip- 
tions of the outcrops of silicic igneous rocks of these counties, while the 
occurrence of gabbro also has been noted. Diorite is mentioned as oc- 
curring in the vicinity of Watab in the western part of Benton County, 



52 



GEOLOGY OF EAST CENTRAL MINNESOTA 



and near Torah (Richmond) and Cold Spring in the southern part of 
Stearns County. Only one group of gabbro exposures is known in this 
region, being located in the northeastern corner of T. 125N., R. 29W., 
and adjacent parts of T. 125N., R. 28W., in Stearns County. It occupies 
portions of several contiguous sections. 80 

AITKIN, CARLTON, AND PINE COUNTIES 

Among the most important rock outcrops in central Minnesota with ref- 
erence to the geology of the Cuyuna district, are those of quartzite, diorite, 
and diabase on the shores of Dam Lake and Long Lake in central Aitkin 
County, about 3 to 6 miles south of Kimberly. There are two quartzite 
outcrops, both located on the northwest shore of Dam Lake, one in sec- 
tion 34 and the other in section 35, T. 47N., R. 25W. They are described 
in connection with the metamorphic rocks. The diabase and diorite out- 
crops are southwest of the quartzite outcrops occurring on both the east 
and west sides of Long Lake in sections 9 and 10, T. 46N., R. 25W. 

The outcrops in section 9 are by far the most extensive, occupying 
an area of 8 or 10 acres west of the southwestern end of Long Lake. 
The rock is dark green in color and is in general fine-grained. Medium 
coarse-grained phases, however, occur as well. All show a distinct dia- 
basic texture. The rock appears to be less silicic in character and darker 
in color than that composing the outcrops east of Long Lake. It con- 
sists of pyroxene, basic feldspar, and green hornblende. Scattered quartz 
grains occur frequently and magnetite is disseminated through it. The 
rock is massive and fresh-looking. 

On the east side of the lake two small groups of outcrops occur ; one 
along the top of an elevation overlooking the northeastern part of the 
lake, and the other one a short distance from the mouth of a small brook 
which empties into the southeastern part of the lake. Both occur in sec- 
tion 10. 

The group of outcrops at the northeastern end of the lake consists of 
three or four exposures, all of them quite small. The rock is medium 
to dark gray in color and massive, resembling diorite in appearance. It 
is medium to fine grained and consists of feldspar and pyroxene with 
some green hornblende and mica and scattered pyrite specks. 

The outcrops on the southeastern side of the lake are on the banks 
of a small brook about 200 feet from its mouth. Two types of rock occur 
here. One is a gray massive rock similar to that occurring at 
the northeastern end of the lake, while the other is a lighter 
gray, schistose, micaceous rock which appears to be an alteration 



w Bowles, O., and Grout, F. F., Unpublished map. 



ROCKS OF EAST CENTRAL MINNESOTA 



53 



product of the massive rock. The schistose rock is found along irregular 
zones in the massive rock, all gradations occurring along the contacts. 
The rock is finely and distinctly laminated, white laminae, probably of 
crushed feldspar, being interlayered with gray laminae containing mica 
and other ferromagnesian minerals. The most conspicuous mineral in 
it is white mica which occurs in tiny flakes parallel to the lamination and 
produces the schistosity. 

The igneous rocks at Long Lake are designated by Van Hise and 
Leith 61 as Keweenawan, and are believed to be intrusive into the upper 
Huronian metamorphosed rocks of this region. Winchell, 62 on the other 
hand, believed that they are equivalent to the Keewatin greenstone of 
northern Minnesota, and that they formed the basement on which the later 
sediments were deposited. The former draw their conclusions from the 
fact that numerous dikes of subsilic igneous rock similar to those occurring 
atXong Lake are found in Carlton County, and elsewhere cutting schists 
and slates of upper Huronian age, while the latter notes the similarity 
between these rocks and the greenstone of the Vermilion district and 
believes that the quartzite at Dam Lake overlies them. 

There are a number of localities in the Cuyuna iron-ore district 
where subsilicic and intermediate rocks have been shown by drilling to 
be interlayered with the series of metamorphosed sedimentary rocks, and 
seem to have been deformed with them. They are evidently not basement 
rocks on which the metamorphosed series was deposited, nor are they 
later intrusives in the series. Such rocks are evidently of approximately 
the same age as the metamorphosed rocks, and bear a relation to them 
similar to that which the Hemlock volcanic rocks bear to the middle 
Huronian metamorphosed sediments of the Crystal Falls district of 
Michigan. 

Subsilicic rocks of this character that have suffered considerable 
deformation and alteration are found outcropping at a number of locali- 
ties in Carlton and Aitkin counties besides the Long Lake area already 
mentioned. Some of these occurrences simply exhibit schistosity while 
in others considerable metamorphism and recrystallization of the minerals 
have taken place. The rock differs distinctly from that which occurs as 
dikes and intrusive masses in the slate occurring along St. Louis River 
or in the schist and slate of the Cuyuna district. While it is not impos- 
sible that post-Keweenawan deformation may have produced changes in 
these rocks, it seems more logical, in view of the fact that igneous rocks 
are known to be associated with the schists of the Cuyuna district, to 



Van Hise, C. R., and Leith, C. K., The geology of the Lake Superior region: U. S. Geol. 
Survey Mon. 52, PI. 14, 1911. 

62 Winchell, N. H., The Cuyuna iron range: Econ. Geology, vol. 2, p. 568, 1907. 



54 



GEOLOGY OF EAST CENTRAL MINNESOTA 



class the more altered types of subsilicic and intermediate igneous rocks 
with this older group. 

C arlton County contains outcrops of subsilicic igneous rocks both of 
the older and the later types in many places. 

In the NWJ4 of SWJ^ of section 19, T. 47N., R. 21W., a short dis- 
tance north and west of Dead Moose River, and just east of the boundary 
line between Carlton and Aitkin counties, are several low exposures of 
subsilicic igneous rock. At the western end the group of outcrops is 
covered for the most part by large boulders. In the central and eastern 
parts, however, the outcrops rise a few feet above the surrounding area 
and slope gradually toward the river. The exposures occupy an area 
about 1.500 feet long east and west, by 400 feet wide north and south. 
The rock along the north and south sides of the group of outcrops is 
medium coarse-grained and of a dull grayish green color. It is made up 
largely of feldspar and hornblende with probably some pyroxene and 
small scattered crystals of magnetite and pyrite. It may be a diorite or 
an altered gabbro. In the center of the group of outcrops between the two 
areas of medium coarse-grained rock, is a dull, dark gray rock with fine- 
grained texture. This rock has suffered considerable alteration so that 
it is difficult to tell its nature without thin sections. It contains consid- 
erable carbonate, and from field relations it appears that the rock is prob- 
ably derived from the alteration of the original diorite or gabbro. In 
the central part of the area of this rock is a light gray, crystalline, car- 
bonate rock made up largely of carbonate and quartz with scattered 
pyrite crystals. Under surface conditions, the carbonate in it alters to 
a hydrous oxide of iron, suggesting that at least part of it is iron car- 
bonate. Quartz veins and pegmatite dikes, vertical or dipping steeply to 
the south and trending about east and west, cut the other rocks. At the 
contact of the medium coarse hornblende-feldspar rock and the fine- 
grained altered rock, some shearing has taken place, giving the rock a 
slightly banded structure. The bands strike N. 56 \Y., and dip steeply 
to the south. 

In the S\\ l i of SEJ4 of section 20, T. 46N., R. 21W., on the land 
of L. Pevon, an outcrop of fine-grained diabase occurs south and east 
of the farm-house. The main outcrop trends a little north of east for 
a distance of about 300 feet, while a smaller exposure which is badly 
decomposed by weathering, occurs about 100 feet farther east. The rock 
is a dark gray or green, massive, fine-grained, altered diabase. Small 
grains of pyrite occur abundantly throughout the mass. Near the center 
of the outcrop a belt of schistose rocks from 5 to 8 feet wide extends 
from north to south. Running parallel with the schistosity are narrow 
white bands of calcite. From the field relations, it appears that shearing 



ROCKS OF EAST CENTRAL MINNESOTA 



55 



has taken place between the two masses of diabase, and this schist is a 
shear zone product. The strike of the schistosity is about N. 40 E. and 
the dip is 67 southeast. At the west end of the outcrop is a quartz vein 
six inches wide striking east and west and dipping steeply to the south. 
On both sides of this vein for a few inches from the contact the rock 
also is schistose. All gradations can be found in the outcrop from the 
schistose phases to the massive diabasic phases. In this respect it re- 
sembles somewhat the exposure occurring on the southeast side of Long 
Lake in Aitkin County. The occurrence of the schist indicates that the 
rock has suffered considerable metamorphism after it was intruded. 

Several outcrops of dark igneous rock occur on both sides of the north- 
south road between sections 14 and 15, T. 46N., R. 21 W. The typical 
rock is a greenish black, very fine-grained diabase, but schistose phases 
occur intermixed with the massive rock. The southernmost exposure 
lies about 60 feet west of the road and about 750 feet north of the south- 
east corner of section 15. It consists mainly of light greenish gray horn- 
blende schist, impregnated with specks of pyrite and containing small 
scattered crystals of feldspar and calcite. The strike of the schistosity 
is N. 70 E., and the dip is about 50 to the south. The next outcrop to 
the north in this group is 50 feet south of the quarter line. The larger 
part of the exposure lies in section 15, but it crosses the road and extends 
a few feet into section 14. It is a dark grayish green, or greenish black, 
dense, fine-grained diabase, made up principally of fine lath-shaped 
crystals of feldspar with associated hornblende, pyroxene, and scattered 
crystals of pyrite and magnetite. The central part of the outcrop is 
slightly sheared, developing an incipient schistosity. The original texture 
is still retained, but it has undergone some alteration along the shear 
planes where an abundance of mica has developed. A quartz vein a foot 
and a half wide with a vertical dip, and trending almost due north and 
south, cuts the west end of the outcrop, while smaller stringers of quartz 
and pegmatitic material are of frequent occurrence. Most of them fill 
north and south joints, but a few run east and west. The remaining out- 
crop in the group consists of two small exposures of rock similar to that 
described above, located about 200 feet north of this on the west side 
of the road, and a larger one situated about 780 feet north of the quarter 
line on the east side of the road. The latter outcrop occurs at the road- 
side and extends eastward for about 300 feet. The rock differs from 
that described above in being more schistose. The schistosity strikes 
about N. 65 E., and dips about 50 to the south. Some phases of this 
rock consist of well developed schist. Scattered through parts of the 
rock are small white spots made up principally of white feldspar, giving 
the rock a mottled appearance. These are elongated parallel to the schis- 
tosity. Along the shearing planes, some of which show slickensides, 



56 



GEOLOGY OF EAST CENTRAL MINNESOTA 



sericite is abundantly developed. This rock, as far as its general appear- 
ance is concerned, is similar to that in the NE^J of of section 4, 
T. 46N., R. 21W., described below. 

Two small outcrops of dark igneous rock occur in the NE*4 of NE*4 
of section 4, T. 46N., R. 21W., on the land of William Michalski. The 
outcrops lie in the cleared field between the road and the house. The 
rock is medium-grained, light grayish green diabase, considerably altered. 
In the easternmost outcrop the rock is massive, while in the outcrop to 
the west the rock has suffered considerable dynamic metamorphism pro- 
ducing a schistose structure. The schistosity strikes about N. 65 E. and 
dips about 45 to the south. The rock contains long, slender plagioclase 
crystals imbedded in a light green to gray, fine-grained ground-mass. 
Occurring abundantly throughout the rock are larger crystals of white 
feldspar largely altered to calcite, giving the rock a porphyritic appear- 
ance. Calcite occurs not only as an alteration of the large crystals, but 
also abundantly in the fine-grained matrix. 

In section 28, T. 47N., R. 20W., about 600 feet due east of the west 
quarter post, an outcrop of highly altered diabase occurs on the face of 
an escarpment which rises about 25 feet above the bottom of the valley 
of Kettle River. The escarpment runs about N. 30 E. and for about 175 
feet the rock is exposed along it. The diabase is massive, dull, grayish 
green and very fine-grained. Calcite occurs as an alteration of the feld- 
spar, but the original diabasic texture is still retained. Besides chlorite 
and hornblende, which are probably mainly alteration products, there are 
small grains and aggregates of magnetite and pyrite. Quartz veins are 
abundant and thin films of quartz occur along the joint planes. 

On the opposite side of Kettle River from the outcrops described 
above, about 100 feet south of the northeast corner of NWJ4 of the 
SWJ4i section 28, T. 47N., R. 20W., two small exposures of rock lie 
at the water's edge on the bank of the river. About 300 feet southwest- 
ward a low escarpment rises 20 feet above the river. Near the edge of 
the latter, also, rock is exposed for a distance of about 125 feet. The 
rock composing these exposures is a dark grayish green, finely crystalline, 
igneous rock, much of which shows a fine ophitic texture. In places the 
rock is dense, while elsewhere it shows a well-developed schistosity. The 
strike of the schistosity is about N. 45 E. and the dip is 70 to the south- 
east. The outcrops in section 28 have the appearance of being composed 
of fine-grained diabase which has locally suffered considerable dynamic 
metamorphism. The rock is very similar to other subsilicic igneous rocks 
exposed in the southwestern part of Carlton County which are judged to 
be flows or intrusive sheets interlavered with the metamorphosed sedi- 
mentary rocks of the region. This has already been discussed in connec- 



ROCKS OF EAST CENTRAL MINNESOTA 



57 



tion with the description of similar rocks found at Long Lake in Aitkin 
County. 

Another outcrop of schistose dark igneous rock occurs about 650 feet 
east of the northwest corner of the NW}4 of the NE*4, section 2, T. 46N., 
R. 20W., and continues southwestward across the forty-acre tract toward 
the north-south quarter line. It lies in a low, flat, glaciated ridge, partly 
covered by a thin mantle of soil. The rock is dark greenish gray, dense 
and fine-grained, and has well-developed cleavage. It is comparatively 
fresh and appears to have a fine diabasic texture. It is similar to that 
occurring in the NE^ of SE%. of section 15, T. 46N., R. 21 W. Several 
sets of joints cut the rock into large polygonal blocks. The cleavage strikes 
about N. 45 E. and dips 17 to the southeast. 

Near the center of section 5, T. 47N., R. 18W., on the south side of 
Park Lake River, two outcrops of dark igneous rock, occupying an area 
about 180 feet long and 120 feet wide, rise 10 to 15 feet above the 
meadow. They form two small knolls elongated in an east-west direc- 
tion, one being north of the other. The rock is dark gray and medium- 
grained, resembling diorite. The principal minerals are feldspar, horn- 
blende, magnetite, and pyrite. The edges of the larger feldspar crystals 
have been somewhat altered. Large cubes of pyrite are scattered abun- 
dantly through the rock. About a quarter of a mile north is an outcrop 
of similar rock which, however, is somewhat schistose and in places is 
mottled due to the presence of white grains of feldspar and calcite. 

Small dikes or sheets of subsilicic igneous rocks occur quite commonly 
in the belt of slate extending along St. Louis River in northern Carlton 
County and thence southwestward through central and southern Carlton 
County, and outcrops of them have been noted in many places. They are 
mentioned in connection with the descriptions of these rocks. One of the 
larger ones of these is an outcrop of much altered diabase which lies in 
the bed of West Branch of Moose Horn River in the SW% of NW}4, 
section 15, T. 47N., R. 19W. It occurs intrusive into graphitic slate. 
Dikes of fresh gabbro and diabase cut the slates and phyllites in the val- 
ley of St. Louis River, being especially abundant near the Thomson dam 
and elsewhere between Carlton and Cloquet. 

Dikes of dark igneous rocks have also been mentioned as occurring 
in various silicic igneous rocks of east central Minnesota, already de- 
scribed, such as those found along Snake River and Hay Creek. 

A large number of outcrops of subsilicic volcanic rocks occurs at 
various points in eastern and southeastern Pine County. They form a 
part of the area of Keweenawan igneous rocks which extends southwest- 
ward from Douglas County, Wisconsin, into eastern Minnesota, forming 
the so-called "south escarpment." The rocks are practically all of eruptive 
origin, consisting of massive diabase with texture varying from medium 



58 



GEOLOGY OF EAST CENTRAL MINNESOTA 



coarse to extremely fine and of various types of amygdaloids. Locally 
tuffaceous layers occur, but they are not abundant. These rocks have 
been mentioned in the section on the general geology of east central 
Minnesota. They bear no direct relation to the rocks of the Cuyuna iron- 
ore district, and need not be discussed further here. They have been 
mapped and described in detail by Grout. 63 

OUTCROPS OF SLATE, SCHIST, AND OTHER METAMORPHIC ROCKS 
MORRISON AND BENTON COUNTIES 

The westernmost of the outcrops of metamorphic rocks in central 
Minnesota are chloritic schists found in the vicinity of Randall. They 
cover an area nearly half a mile long north and south, and about 500 feet 
wide. The north end of this area of outcrops is at the railway station, 
the rock being exposed in a cut nearly opposite the station. The outcrops 
are not continuous but occur at frequent intervals, southwest of the rail- 
road and of the north branch of Little Elk River. 

The chloritic schist is dark green and is essentially similar in all the 
exposures. It is commonly fine-grained and shows a marked but uneven 
cleavage which has a general direction of N. 40 E. and has dips varying 
from vertical to 70 northwest. No bedding is distinguishable. 
(Plate VI.) 

The rock is essentially a fine-grained mixture of chlorite and feldspar 
with some quartz, which varies in abundance in different parts. Both 
chlorite and feldspar show parallel arrangement and elongation along 
the general direction of the cleavage. The quartz is in irregular grains 
either disseminated with the other minerals or bunched in spots. Through 
this fine-grained matrix are scattered somewhat larger grains of siderite. 
These vary in size from mere specks to perhaps one-tenth of an inch in 
diameter, and are abundant in all phases of the rock. Some have rhombic 
outlines and consist of single crystals, but most of them are irregular 
and consist of several individual crystals. Near the surface the siderite 
is oxidized to limonite and forms yellowish brown specks of ocher. Locally 
also the ocher is concentrated along cracks and in small openings. In 
some phases of the rock siderite in small irregular grains is disseminated 
through the matrix, along with feldspar, chlorite, and quartz. 

Besides grains and little bunches of siderite, other minerals such as 
quartz, magnetite, and chlorite are found in the rock. Magnetite is very 
abundant locally, and although it is generally distributed through the 
rock it may be entirely absent in some places. Some magnetite grains 
have crystal outlines, while others are irregular. Quartz usually does 
not occur in large grains, but bunches of small grains are numerous. 



••Grout, F. F., Contribution to the petrography of the Keweenawan: Jour. Geology, vol. 

18, pp. 633-657, 1910. 



PLATE VI 




B. CHLORITIC SCHIST OUTCROPS AT RANDALL SHOWING CLEAVAGE 
PHOTO BY CARL ZAPPFE 



ROCKS OF EAST CENTRAL MINNESOTA 



59 



Chlorite generally is evenly disseminated but locally it occurs in bunches. 
Small grains of muscovite are found disseminated in much of the rock. 

While all the rock is chloritic schist, there are local minor variations 
in the composition, color, and texture, as already indicated. At several 
points, nodular light-colored brownish masses are found which consist 
of quartz, feldspar, and siderite, but contain no chlorite or magnetite 
while elsewhere magnetite grains are closely spaced. The variation in 
magnetic attraction in the vicinity of Randall is attributed to this abund- 
ance of magnetite in the chloritic schist. 

The Randall exposures are directly on the line of the southwest ex- 
tension of the Cuyuna range. The rock is probably continuous with the 
chlorite schists which have been found to be so abundant in association 
with the iron-bearing formation in various parts of the Cuyuna range. 
In general appearance and composition the rock at Randall resembles 
closely the chloritic schist found in many of the Cuyuna range mines. 

In section 3, T. 130N., R. 30W., about three miles east of Randall 
there are a few small exposures of chloritic schist which might readily be 
mistaken for boulders. They are located in the woods near the center of 
the section, just east of a little-used north and south wagon road that 
runs along the quarter line. 

While most of the outcrops consist of dark green or grayish green 
chloritic schist, some of them have small bands of a gray porphyritic 
rock with large phenocrysts of white feldspar. Such bands are enclosed 
between chloritic schist, and there is a complete gradation between the 
two, showing that the chlorite schist is a dynamically metamorphosed 
phase of the porphyry. The direction of the cleavage is similar to that 
in the exposures near Randall. 

About two miles north of Little Falls, Little Elk River empties into 
Mississippi River. Outcrops of gray slate and phyllite occur on both 
sides of this stream, the easternmost of the exposures being about 500 
feet above its mouth. They extend along the stream for about 700 feet 
and within this distance several folds are found. The outcrops show 
beautifully both cleavage and bedding, and the relation between the two 
can be determined in the individual outcrops. The general strike of the 
bedding is about N. 50 E., while the dip varies on account of the fold- 
ing. The strike of the cleavage varies between N. 32 E. and N. 61 E. 
The dip of the cleavage planes varies in the individual folds, but is in 
general about 75 southeast. 

The exposures consist mainly of finely crystalline phyllite, most of 
which shows fine lamination parallel to the bedding. The rock is made up 
of fine quartz and biotite which vary in relative abundance in different 
layers. In some of the exposures, massive graywacke beds are inter- 
layered with finely laminated, more highly micaceous phyllite beds. All 



6o 



GEOLOGY OF EAST CENTRAL MINNESOTA 



the beds show cleavage, but the micaceous layers show it more perfectly. 
Thus the relation between the cleavage and the axial planes and pitch of 
the folds can be readily seen, and the relation of these folds to the major 
structure determined. The pitch of the folds is to the southwest, while 
the axial planes dip to the southeast, indicating that the folds are situated 
on the south limb of a major anticline. 

In the city of Little Falls, a small island (Mill Island) occurs in Mis- 
sissippi River just below the dam. ( Plate VII A.) The shores around 
the northern half of this island consist of continuous outcrops of slate and 
phyllite. most of which are gray and finely micaceous. Although con- 
siderable recrystallization has occurred, the bedding is still plainly visible 
both on account of fine lamination and on account of an interlayering of 
beds of different materials. Some beds are quartzose while others are 
argillaceous and micaceous, the latter, however, being much the more 
abundant. 

The beds are flexed into a number of minor folds all trending in a 
general northeast-southwest direction diagonally across the island. The 
strike of the bedding is approximately X. 30 E., parallel to the folds, 
while the dip varies. The schistosity is fairly regular, in general striking 
approximately parallel to the bedding and dipping at angles varying be- 
tween 75 NW. and 75- SE. The relation of the cleavage to the bedding 
is clearly shown in some of the minor folds. ( Plate VII B.) 

The rock on Mill Island is principally dark gray, finely micaceous 
slate and phyllite. some portions of which are argillaceous and some 
quartzitic, the different phases being interlayered. Some beds are mas- 
sive and quite uniform in texture, and show the bedding only by fine 
color lamination, while in other beds thin layers of fine argillaceous slate 
are interbedded with more coarsely crystalline quartzitic or micaceous 
phyllite. The cleavage cuts across all the laminae indiscriminately. The 
rock is composed of biotite and quartz with probably a considerable per- 
centage of white mica in fine sericitic flakes. In the fine-grained slaty 
layers, sericite is very abundant, and distinctly visible mica flakes are 
almost absent, while in the coarser layers numerous fine flakes of biotite 
occur, disseminated in a quartzose or sericitic ground-mass. Other min- 
erals which occur locally in the slate and phyllite are ottrelite, garnet, 
amphibole, and chlorite. Ottrelite occurs as small, scattered, thin, shiny 
black plates lying parallel to the cleavage in some of the phyllite layers, 
and garnet also occurs locally in the phyllite in tiny, disseminated crystals. 
Both are rare. Garnet, amphibole, and chlorite, however, are found 
more characteristically in a number of elongated, lens-shaped, concretion- 
like masses which occur interlayered with the slate and phyllite. Some 
of these masses are five or six feet long and a foot or more thick. They 
consist of a hard, dark, siliceous ground-mass in which are embedded 



PLATE VI 




B. SLATE OUTCROP AT LITTLE FALLS SHOWING THE RELATION OF CLEAVAGE TO BEDDING 



ROCKS OF EAST CENTRAL MINNESOTA 



Oi 



abundant fibrous, black amphibole or dark red garnet with associated 
dark green flakes of chlorite. The amphibole crystals range up to one 
half of an inch in length and show no regularity in their arrangement. 
The garnet crystals average about one eighth of an inch in diameter, 
being much larger than those disseminated through the phyllite. Chlorite 
is commonly associated with it, and appears to be an alteration product 
of it. Rarely, garnet and amphibole are found together in the same con- 
cretion-like lenses, but commonly they are in separate lenses. Ottrelite 
also is often distinctly associated with these lenses. It does not occur 
within them, however, but in the phyllite or slate layers along their bor- 
ders. Amphibole, on the other hand, is found only in the concretion-like 
masses, and not in the slate and phyllite. The origin of the concretion- 
like masses has not been determined. 

Another outcrop of gray slate and phyllite similar to that at Little 
Falls is found on Swan River near Ledoux in the SE*4 of SE}4 of sec- 
tion 4, T. 128N., R. 30W., about 7 miles southwest of Little Falls. 
The exposures are on both sides of the stream near the water level and 
are entirely covered during high water. They are found just east of the 
bridge between sections 3 and 4. The rock is a finely crystalline mica- 
ceous, argillaceous, or quartzitic phyllite with well-developed cleavage 
striking about N.20°E., and dipping from 57 southeast to vertical. The 
bedding is shown by fine banding and lamination which is cut across by 
the cleavage lines. It strikes in general N.25°E. and dips 75°N. 

L T pham 64 mentions the occurrence of another outcrop of slate on Swan 
River in section 1, T. 128N., R. 20W., about 2 l / 2 miles below the outcrop 
just mentioned. It is said to crop out in the bed of the stream but was 
not found by one of the writers (Johnston) who visited the locality. 

Scattered outcrops of garnetiferous, staurolitic mica schist are found on 
both banks of Mississippi River at intervals between the mouth of Swan 
River, about 4 miles south of Little Falls, and a point about one half 
mile south of the Minneapolis, St. Paul, and Sault Ste. Marie rail- 
road bridge in section 32, T. 128N., R. 29W. The best exposures are 
on the west bank, but good exposures occur on the east bank also. 
Locally they extend partly or entirely across the river, forming rapids. 

The exposure farthest north is on the west bank of the river at Pike's 
Rapids, a short distance below the mouth of Swan River in section 7, 
T. 128N., R. 29W. Southward from this locality exposures are found 
at Cash's Rapids in section 20, T. 128N., R. 29W. and at various points 
between Cash's Rapids and the railroad bridge above mentioned. Below 
the bridge several exposures occur as far south as Blanchard's' Rapids, 



64 Upham, Warren, The geology of Crow Wing and Morrison counties: Geol. and Nat. Hist. 
Survey of Minn., Geol. of Minn., vol. 2, 1882-1885, p. 600. 



62 



GEOLOGY OF EAST CENTRAL MINNESOTA 



where are found the outcrops of gabbro, already described. (See page 51.) 
In one small outcrop about 1,000 feet south of the bridge, a very quartz- 
itic phase of the schist is cut by a narrow dike of dark hornblende rock. 

Most of the schist exposures are near the river's edge, and very few 
extend far above the level of the water on the river bank. At one or two 
localities, however, as at the Minneapolis, St. Paul, and Sault Ste. Marie 
railroad bridge, fairly large exposures are found. Because of the highly 
metamorphosed state of the rocks, it is only in the larger exposures that 
it is possible to determine the attitude of the bedding and its relation to 
the schistosity. Thus at the railroad bridge the schist, although strongly 
metamorphosed, shows interlayering of coarsely crystalline micaceous 
phases with finer grained quartzitic phases indicating bedding. The 
layers in this exposure strike N.8°E. and dip 57°NW. while the schis- 
tosity strikes N.i5°E. and dips 52°NW. In the outcrops at the other 
localities visited, it was not possible to distinguish the bedding. The 
schistosity, however, was found to be perfect and fairly regular. Thus 
at Pike's Rapids, below the mouth of Swan River, the strike of the cleav- 
age is N.30°E. and the dip is 65°N W., while at Cash's Rapids the strike 
is N.30°E. and the dip 6o°NW. Above Blanchard's Rapids, south of 
the railroad bridge in the last schist exposure downstream, the strike of 
the schistosity is about north and south and the dip is 65°W. Thus 
there appears to be a gradual turning of the direction of schistosity from 
a strike of N.30°E. to nearly north and south on going southward. 

The schist in all the exposures is very similar, being a micaceous or 
quartzose micaceous schist with abundant crystals of garnet and stauro- 
lite scattered through it. The schist, while in general similar to that 
at Little Falls, is much more coarsely crystalline and is more uniform in 
character, consisting of intermixed quartz grains and biotite flakes, the 
latter parallel to the schistosity. The fine interlayering which is common 
in the slate and phyllite at Little Falls has here been entirely obliterated, 
due to recrystallization. Garnet and staurolite are both abundant and 
occur throughout the rock. The garnet crystals are small, usually aver- 
aging about one twentieth of an inch in diameter, but the staurolite crys- 
tals range up to an inch in length and one half inch in width and 
frequently show cross-shaped twins. The staurolite and some of the 
garnet also contains abundant included quartz grains and occasional bio- 
tite flakes. The outline of the staurolite crystals as seen under the micro- 
scope is very irregular and indicates that the staurolite has grown at the 
expense of the quartz and biotite. The quartz grains included in the 
staurolite crystals are smaller than those in the remainder of the rock, 
showing they have probably been partly resorbed to form staurolite. A 
few staurolite crystals contain included garnet crystals also. Ottrelite 
which is present in the rocks at Little Falls has not been found in these 



ROCKS OF EAST CENTRAL MINNESOTA 



63 



schists. Probably it has disappeared during the more intense metamor- 
phism which the schists have suffered. The paragenesis of minerals in 
the formation of garnetiferous and staurolitic schist from the original 
shale appears to be in the following order: quartz, biotite, ottrelite, gar- 
net, staurolite. 

It seems beyond question that the slate and phyllite on Little Elk 
River and at Little Falls, and the garnetiferous and staurolitic schists on 
Mississippi River represent different stages in the metamorphism of the 
same original rock. The metamorphism seems to have become more 
intense on going southward, due probably to the presence of the great 
granite mass in the St. Cloud region against which the sediments were 
crushed. This fact would seem to- indicate that much of the granite in 
this region was present before the deformation of the sediments took 
place, probably being pre-Keweenawan in age. The alternative hypoth- 
esis is that the granite is later in age, and that the metamorphism of the 
sediments was due to the intrusion of the granite. It does not seem 
likely, however, that the metamorphic effect of such an intrusion should 
be felt for many miles away from the contact, especially in an argillaceous 
sedimentary rock. Besides, the minerals developed are characteristic of 
dynamic metamorphism and not of contact metamorphism. 

Hall 65 notes the probable occurrence of schist in the northern part 
of Benton County, and on the basis of this Van Hise and Leith 66 have 
extended the upper Huronian slate area into Benton, Sherburne, and 
Stearns counties, while the granitic rocks of this region have been mapped 
as Keweenawan intrusives. The data on which the mapping of this 
schist was based are not available, so that the character and extent of 
the schist are not known. 

CROW WING, CASS, AND AITKIN COUNTIES 

In the region north of the Cuyuna district and west and southwest of 
the Mesabi district, no rock outcrops are definitely known, although sev- 
eral have been reported from different localities. 

An outcrop of crystalline limestone is frequently mentioned as occur- 
ring in NW^4 of SE^ of section 29, T. 137N., R. 28W. south of White 
Fish Lake in northern Crow Wing County. This supposed outcrop has 
been visited by the writers, who have come to the conclusion that it is 
a large glacial boulder. The rock rises about 15 feet above the ground 
and is about 20 feet by 30 feet at the base. It has a rather jagged out- 
line and consists of light gray to pink crystalline limestone with numerous 
small masses and thin layers of chert. Certain layers are somewhat 

66 Hall, C. W., Keewatin area of eastern and central Minnesota: Bull. Geol. Soc. Amer., 
vol. 12, pp. 343-376 (map), 1901. 

w Van Hise, C. R., and Leith, C. K., op. ext., Pis. I and XIV. 



64 



GEOLOGY OF EAST CENTRAL MINNESOTA 



argillaceous and serieitic, others are quartzitic. Drilling operations not 
far from this locality indicate that the glacial drift in this region varies 
in thickness from 200 to 300 feet. It appears improbable that there 
should be such extreme irregularities as would result in the occurrence 
of rock outcrops. 

Occasional references are seen in the literature 07 to an outcrop of rock 
in Cass County, reported as occurring along the northern edge of T. 
142N., R. 27W., on Boy River, one or two miles below Boy Lake. The 
lithological character of the rock is not known, but it is presumed to be 
quartzite. The writers have not seen the outcrop, nor have they seen any 
person who knows definitely of its existence. 

Quartzite outcrops are found near Dam Lake, Aitkin County, at two 
localities, one in the SE*4 of section 34, and the other in the NW^4 of 
section 35, T. 47N., R. 25W. The rocks composing the exposures are 
very similar in both localities, being hard, coarse, light gray to pink vit- 
reous quartzite. Much of the rock is very coarse, being fine conglomerate 
or grit rather than quartzite. The particles composing it range up to 
one fifth of an inch in diameter, and all are distinctly rounded. They con- 
sist almost entirely of glassy quartz, some being perfectly transparent and 
colorless while others are bluish white opaline. They are all firmly 
cemented so that the rock breaks across the fragments rather than 
around them. 

At both localities, several small rounded exposures protrude just 
above the soil. The rock is very massive and shows no trace of bedding. 
Even banding due to interlayering of coarser and finer material is appar- 
ently absent. A few joints cut across the outcrops and some of them 
may be separation planes indicating the bedding, but their character is 
very doubtful. Winchell, 08 who visited these outcrops, states that the 
exposure in section 34 shows an unmistakable bedding plane striking 
N.6o°E. and dipping S.30°E. at an angle of 25 . This observation was 
not confirmed by an examination of the outcrops made by the writers. 
The line which connects the two outcrops, however, and the extension 
of which to the southwest passes over other occurrences of quartzite 
found by drilling, has a direction of N.5o°E. which corresponds fairly 
well with the general strike of the sedimentary beds throughout the 
Cuyuna district. The general dip of these beds also is to the southeast, 
but at a very steep angle. It does not seem improbable, therefore, that 
the quartzite bed has a general northeast-southwest strike and a south- 



m Upham, Warren, The geology of Cass County and of the part of Crow Wing County 
northwest of the Mississippi River: Geo!, and Nat. Hist. Survey of Minn., Geol. of Minn., vol. 4, 
1 896- 1 808, p. 65. 

•Winchell, N. H. t The Cuyuna iron range: Econ. Geology, vol. 2, pp. 566-567. 



ROCKS OF EAST CENTRAL MINNESOTA 



65 



easterly dip, although there is apparently nothing in the individual ex- 
posures to indicate this structure. 

PINE, CARLTON, AND SOUTHERN ST. LOUIS COUNTIES 

Several large areas of outcrops of metamorphosed sedimentary rocks 
are found in the northwestern part of Pine County, south, east, and north- 
east of Denham, these being the southernmost outcrops of this type of 
rock in eastern Minnesota. The rocks are principally mica schists, but 
hornblende schist, quartzite, and limestone occur also. 

The largest area of outcrops is that through which the Minneapolis, 
St. Paul, and Sault Ste. Marie Railroad passes east and northeast of 
Denham. The principal exposures are in the eastern half of section 19, 
T. 45N., R. 20W., but some are found also in the western part of sec- 
tion 20 and in the southeastern part of section 18. The railroad cuts 
through or passes over most of the outcrop. 

The rock throughout this area is quite uniform, being a quartzose 
mica schist. It varies slightly, however, in texture and in the relative 
amounts of mica and quartz which it contains in different parts. Locally 
it is fine-grained and quartzitic, consisting of sugary quartz with abundant 
fine scattered biotite flakes parallel to the schistosity. Elsewhere the 
quartz is present in minor amount and the mica is more coarsely crystal- 
line, producing a very marked schistosity. Local layers consist almost 
entirely of mica. Small disseminated crystals of garnet are scattered 
throughout the rock, but are more abundant in the coarsely micaceous 
phases. The rock in general is dark gray, the mica being mainly biotite ; 
but white mica is intermixed with it. 

Schistosity is strongly developed throughout the area so that in gen- 
eral it obliterates the bedding. Locally where the bedding can be dis- 
tinguished by the faint interlayering of the quartzitic with the more 
coarsely micaceous phases, it appears to be in general parallel to the 
schistosity. The schistosity is quite regular in strike in the different out- 
crops throughout the area, being about east and west or slightly north of 
east. The dip, however, varies considerably in different places. In the 
southern part of the area it is to the north at angles varying in general 
between 30 and 70 . In the northern part of the area, however, both 
northerly and southerly dips are found. Locally the schistosity is almost 
horizontal. 

About a mile southeast of Denham is one of the most interesting 
groups of outcrops of metamorphosed sediments in east central Minne- 
sota. The exposures occur principally in the Ej/2 of SE^4 of section 25, 
T. 45N., R. 21W., but extend for short distances into the adjacent parts 
of section 30 on the east and section 36 on the south. An irregular de- 
pression with low bluffs along it runs in a southeasterly direction through 



66 



GEOLOGY OF EAST CENTRAL MINNESOTA 



this area. It varies from 200 feet to 400 feet in width, and has the ap- 
pearance of being a small abandoned stream valley. The rock outcrops 
are found along the low bluffs on both sides of the depression. The area 
is wooded, the center of the depression being swampy. 

The most northerly outcrops on both sides of the depression consist 
of hornblende schist. To the south of these, a large outcrop of quartzite 
is found on the west side and an outcrop of mixed quartzite and crystal- 
line limestone on the east side. South of the quartzite on the west side 
are exposures of hornblende schist, followed by more quartzite and 
finally mica schist, while on the east side opposite to these are outcrops 
of mica schist and of a dark fine-grained igneous rock. The strike of the 
beds throughout is approximately east and west, while the dips vary 15 
or 20 on either side of vertical. The dip of the cleavage of the schistose 
rocks is in general to the north at somewhat lower angles. In the schis- 
tose rocks the recrystallization has almost entirely obliterated the bedding, 
but in the quartzite and limestone the bedding is well marked. Locally 
rocks of different character are found interlayered. 

The principal exposures consist of quartzite which has suffered con- 
siderable metamorphism. It is light-colored gray, pink, or brown and 
is medium coarse-grained. Tiny pebbles, generally of light blue quartz, 
are abundantly scattered through the rock, in some places making up 
the larger part of the mass and forming a fine conglomerate or grit. They 
range in diameter up to about one fifth of an inch, although locally peb- 
bles an inch and a half or more in diameter are found. In places fine 
and coarse material occur interlayered in thin beds. The quartzite has 
undergone considerable recrystallization so that abundant muscovite is 
developed throughout. Some parallel arrangement is noticeable in the 
muscovite, but this is not prominent enough to produce a marked schis- 
tosity. The muscovite, although generally fine, occurs in distinct flakes. 

The crystalline limestone is associated with the quartzite and re- 
sembles it in appearance. It is pink to light gray in color and finely 
crystalline. Muscovite occurs in it abundantly in fine disseminated flakes 
or is segregated into thin discontinuous streaks. Some phases of the 
limestone are dark gray and contain thin layers consisting largely of 
dark hornblende. The laminae of both muscovite and hornblende are 
developed parallel to the bedding, suggesting that they were formed by 
the recrystallization of impure limestone layers. 

The hornblende schist is dark grayish green and is generally coarsely 
crystalline when composed mainly of hornblende and finely crystalline 
when it contains considerable quartz. It has a well-developed cleavage 
which is especially prominent in the more coarsely crystalline phases. 
The hornblende is dark green and fibrous. Biotite is generally associated 
with it and in places muscovite also occurs. 



PLATE VIII 




B. OUTCROP OF SLATE AND PHYLLITE ALONG THE RAILWAY NORTHEAST OF DENHAM, 
PIXE COUNTY, SHOWING PARALLEL JOINTING 



ROCKS OF EAST CENTRAL MINNESOTA 



67 



The mica schist is light to dark gray. In places it consists mainly of 
muscovite with subordinate quartz, and then shows marked cleavage and 
fine lamination parallel to the cleavage. The cleavage faces present a 
silvery sheen due to aggregates of parallel muscovite flakes. In other 
places, quartz predominates while both muscovite and biotite occur in 
abundant fine flakes. In the latter phase the bedding is still frequently 
recognizable, cutting across the schistosity or parallel to it. The musco- 
vite schist is commonly associated with the crystalline limestone beds, 
frequently being interlayered with limestone. The quartzose mica schist 
with abundant biotite is similar to that exposed along the railway east 
of Denham. 

In Carlton County, abundant outcrops of slate, graywacke, phyllite 
and mica schist occur in the valleys of Kettle, Moose, and St. Louis 
rivers and their branches, the area of outcrops extending in a general 
northeast-southwest direction across the county. Many exposures are 
found directly on the banks of streams, having been cut into by the 
streams during the process of erosion. Other exposures, however, occupy 
higher areas, in places forming prominent ridges rising above the gen- 
eral level and elsewhere occurring as low, flat outcrops. The ridge-like 
exposures are very characteristic. In some localities, as at Carlton and 
near Atkinson, many parallel rock ridges cover considerable areas. Some 
of these are 40 or 50 feet high and many hundred feet long. Nearly all 
of them have a general direction ranging between east-west and northeast- 
southwest, roughly parallel to the cleavage. 

The most southwesterly outcrops of metamorphic rocks in Carlton 
County are those along Split Rock River, a western tributary of Kettle 
River. A high cliff of mica schist about 900 feet long is exposed on the 
south side of Split Rock River in the NW% of NE>4, section 29, T. 
46N., R. 21W. It rises abruptly from 5 to 30 feet above the stream, 
trending approximately east and west. Small exposures appear at in- 
tervals west of the main cliff in the bed of the stream for a distance of 
several hundred feet. The rock is made up of alternate bands of fine 
and coarse grained quartz mica schist associated with some dark graphitic 
schist. Small grains of pyrite and quartz lenses occur in places. The 
finer grained varieties are finely plicated and consist mainly of mica, 
while the coarser grained varieties are rich in quartz. The average strike 
of the schistosity is about N.8o°E. and the dip varies, due to minor folds, 
but is in general to the south at a low angle. The bedding where it can 
be observed is approximately parallel to the schistosity. 

An outcrop of mica schist and phyllite similar to that described above 
occurs in the SE}i of SE}4 of section 21, T. 46N., R. 21W. on the 
south side of Split Rock River. The outcrop begins at the east line of 



68 



GEOLOGY OF EAST CENTRAL MINNESOTA 



the section and extends westward about 1,100 feet. The eastern part 
appears in scattered patches above the soil and the western half stands 
as a vertical cliff from 2 to 10 feet above the water. The bedding in this 
outcrop is well marked because of the alternation of more or less massive 
layers rich in quartz, with finely plicated layers consisting principally of 
mica. The strike of the bedding is about N.75°E. and the dip 30 to 
the south. Locally minor folds are found, which pitch westward at a 
low angle, and the axial planes of which dip southward indicating that 
they are drag folds on the south limb of a larger anticline. 

The outcrops along Split Rock River are the only exposures of schist 
or phyllite found in Carlton County west of the main valley of Kettle 
River. On Kettle River itself abundant outcrops are found at intervals, 
almost from its source, southward through southern Carlton County and 
into Pine County. The most northerly of these outcrops is said to occur 
near Kettle Lake. It is reported that slate was encountered during the 
construction of the drainage ditch heading out of Kettle Lake in the 
SWJ4 of SE T 4 of section 18, T. 48N., R. 19W. The slate is said to 
have been found in the bottom of the ditch a short distance from the 
lake. This has not been verified by the writers. 

The first exposure south of Kettle Lake in the Kettle River valley is 
found in section 2, T. 47N., R. 20W., in the bottom and along the sides 
of the drainage ditch. In the SWJ4 of NEI4 of this section, small ex- 
posures are found in the bottom of the ditch for a distance of about 500 
feet, while in the of SW T 4, beginning about 150 feet southwest 

of the center post and bearing a little west of south, rock is exposed con- 
tinuously in the ditch and along the sides for about 700 feet. The maxi- 
mum width of the exposure east and west is about 800 feet. 

The rock in both localities is made up of alternate bands of slate and 
graywacke. The slate is dark gray to greenish black, very fine grained, 
with well-developed cleavage. The bedding is brought out clearly by 
difference in color due to the variation in the fineness of the material. 
The graywacke is light gray and fine-grained, with a few small scattered 
pyrite grains. The general strike of the bedding is about N.75°E. and 
the dip varies in short distances, due to the presence of minor folds which 
pitch at low angles eastward. Quartz stringers occur along the crests 
and troughs of the minor folds and die out along the limbs. The strike 
of the cleavage is approximately parallel to the bedding and the dip is 
73 to the south. The relations of the folds and cleavage indicate that 
this outcrop is on the south limb of a larger anticline. 

Near the south line of the SE*4 of SWj4 of section 21, T. 47N., 
R. 20W., two long, narrow, parallel rock ridges rise on the west bank of 
Kettle River and trend southwestward. The larger or south ridge is 60 



ROCKS OF EAST CENTRAL MINNESOTA 



00 



feet wide by 275 feet long and rises about 20 feet above the stream. The 
north ridge is about 10 feet wide and 50 feet long, and is only a few feet 
above the stream. The rock is a dark gray, fine-grained, dense horn- 
blende schist which weathers to a rusty brown. Locally it has a marked 
schistosity, and fine mica flakes are developed along the cleavage planes. 
The strike of the schistosity is about N.20°E. and the dip is 8o° south- 
eastward. Much of the fresh rock has the general appearance of being 
dynamically metamorphosed, fine-grained, dark igneous rocks, but the 
original textures have been obliterated and are not recognizable in the 
field. 

The next outcrop of schist to the south in the Kettle River valley is 
near the mouth of Silver Creek- in the SW*4 of NW*4 of section 16, 
T. 46N., R. 20W. About 30 feet from the mouth, in the bed of the 
brook, a light gray, fine-grained phyllite crops out and is cut by a quartz 
vein trending a little east of north and standing almost vertical. This 
was explored for gold about 30 years ago, and a pit 10 feet by 10 feet 
was sunk on it to a depth of about 30 feet. 

The formation outcrops again on the west bank of Kettle River about 
130 feet below the mouth of Silver Creek, making a vertical cliff 15 to 
20 feet high for a distance of about 140 feet along the river. The bed- 
ding is brought out very clearly by the difference in the coarseness of 
the original sediments, giving lighter and darker bands. While most 
of the rock is light gray, fine-grained micaceous phyllite, portions of it 
are highly calcareous and other portions more siliceous. On the weathered 
surface, the interlayering of calcareous portions with siliceous portions 
is clearly shown. The general strike of the bedding is about N.7o°E. 
and the dips vary from 9 to 17 to the south, due to minor folding. 
These minor folds pitch eastward at an angle of about 15 . 

Somewhat over a quarter of a mile below the above outcrop, or about 
the west central part of the NEj/ of SW*4 of section 16, T. 46N., R. 
20W., on the west bank of Kettle River, near the water's edge, is an 
exposure of light yellowish gray, finely laminated micaceous phyllite. 
The rock is badly weathered and soft. It is exposed for about 100 feet. 
The strike of the schistosity is about N.45°E. and the dip varies. At the 
south end of the outcrop it is 25°N., while in the central and northern 
parts it is 45 to the south. 

At this locality, an outcrop of the so-called "Hinckley" sandstone is re- 
ported, 69 but was not found by the writers. 

In the NE*4 of section 22, T. 46N., R. 20W., several outcrops of dark, 
fine-grained hornblende schist occur as ridges on both sides of Gillespie 



w Winchell, N. H., The geology of Carlton County: Geol. and Nat. Hist. Survey of Minn., 
Final Rept., vol. 4, 1896-1898, PI. 56 and p. 16, 1899. 



70 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Brook. At the center of this quarter section, on the north side of the 
brook, is a large ridge trending about east and west. It is about 250 feet 
wide and 800 feet long, and rises from 5 feet to 25 feet above the stream. 
In part it is covered by a thin mantle of soil and loose blocks. The rock 
is greenish gray, fine-grained, sericitic, hornblende schist. Small specks 
and blotches of sericite cover the cleavage surfaces. The schistosity is 
well developed throughout, striking about N.54°E. and dipping 35°SE. 

About 600 feet north of this outcrop several small low ridges of the 
same kind of rock crop out. The strike of the schistosity is N.75°E. 
and the dip is I9°S. About 400 feet southwest of the center of the 
quarter section on the southwest side of the stream a triangular ridge 
rises from 6 to 15 feet above the stream. The rock is the same greenish 
gray schist. The schistosity strikes N.4i°E. and dips 3 southeast. On 
the south side of this ridge is a fine-grained, gray, mottled rock with 
brown specks roughly parallel to a poorly developed schistosity. East 
of the above-mentioned outcrop on the south side of the brook, a long 
low ridge begins about 200 feet from the east line of the section and con- 
tinues eastward parallel with the stream for about 300 feet in section 23. 
The ridge rises a few feet above the stream and is, for the most part, 
covered with soil. On the weathered surface the outcrops consist of a 
rusty brown, micaceous, schistose rock. The fresher material, however, 
is fine-grained gray schist with small rusty brown spots parallel to the 
schistosity. The strike of the schistosity is about N.75°E. and the dip 
I9°S. The rocks composing the group of outcrops in sections 22 and 23 
are mainly fine-grained hornblende schists. All show well-developed 
cleavage and banding. They have somewhat the appearance of being 
dynamically metamorphosed dark igneous rocks, but the original texture 
is no longer recognizable and they are therefore grouped with the schists 
and slates. They resemble closely, however, many of the metamorphosed, 
fine-grained, dark, igneous rocks found in this part of Carlton County 
and may represent a more advanced stage of metamorphism. 

In the SWJ4 of SE*4 of section 21, T. 46N., R. 20W., about a quar- 
ter of a mile north of the mouth of Gillespie Brook, on the east bank of 
Kettle River, an outcrop of decomposed schist makes a cliff 10 to 30 feet 
above the stream, for a distance of about 400 feet. The rock is a badly 
weathered schist, reddish and yellowish green in color and very soft. 
It is broken into small irregular blocks by several intersecting sets of 
joints. The rock shows a fine lamination or color banding parallel to 
prominent, gently undulating, parting planes. Small flakes of sericite and 
quartz stringers lie parallel to the lamination. The color bands may be 
due to the original difference in the material of different layers, or it 
may be secondarily developed parallel to the parting planes. The strike 



ROCKS OF EAST CENTRAL MINNESOTA 



7i 



of the parting planes varies from N.85°E. at the north end of the out- 
crop to N.6o°E. at the south end, and the dip from 9°N. at the north end 
to 25°SE. at the south end. 

About 700 feet south of the above exposure, on the west bank of the 
river, decomposed schist crops out in a vertical cliff about 20 feet high. 
The exposure extends for 150 feet along the stream. The rock is like that 
described above. At the foot of the cliff, however, it is less weathered 
and consists of light greenish gray schist. Near the top of this cliff is a 
rusty light green schist infiltrated with limonite. The rock shows un- 
dulating parting planes similar to those in the outcrop on the east bank 
of the river. 

Just below the mouth of Gillespie Brook, Kettle River turns west- 
ward. About 500 feet west of this bend on the north side of Kettle River 
a low rock ridge can be traced by frequent small exposures from the 
bank of the river northward about 300 feet to an east-west road which 
crosses the river below Gillespie Brook. A small outcrop lies in the 
road about 125 feet farther west. On the river bank the rock forms a 
cliff about 15 feet high and is exposed for a distance of about 250 feet. 
Most of the rock in this exposure is the same rotted, light, rusty green 
schist as that occurring along the river above the mouth of Gillespie 
Brook, although some of the less decomposed material is clearly chloritic 
and sericitic schist. At the west end of the outcrop, however, a different 
type of rock occurs. It is not finely laminated and shows no color band- 
ing, but is a soft, decomposed, light yellowish green, fine-grained rock 
with conchoidal fracture. Judging from its structural relation to the 
schist, it is probably a dike or sill of decomposed igneous rock. The 
contact of this rock and the schist strikes N.40°E. and dips 47°SE., while 
the banding of the schist strikes about N.34°E. and dips 25°SE. The 
exposures in the road consist of decomposed, greenish brown schistose 
rock which appears to have been chloritic schist. The schistosity strikes 
about N.70°E. and dips 28°S. 

The only other outcrop of greenish brown rotted schist occurring in 
this vicinity forms a bank 15 feet high on the south side of the above- 
mentioned road about 350 feet east of the mouth of Gillespie Brook, in 
the NWJ4 of NE54 of section 28, T. 46N., R. 20W. The strike of the 
schistosity is about N.70°E. and the average dip is about 50°S. 

Black graphitic schist outcrops in a low mound about 600 feet south- 
west of the mouth of Gillespie Brook and about 300 feet from the south 
bank of Kettle River, in the NEJ4 of NW/ 4 of section 28, T. 46N., R. 
20W. This mound is about 50 feet wide east and west and about 80 
feet long. A small cut 10 by 20 feet, and 6 feet deep at the face has been 
excavated in it. The rock has well-developed schistosity and shows 



72 



GEOLOGY OF EAST CENTRAL MINNESOTA 



crumpled and crenulated foliation planes along which graphite is devel- 
oped abundantly. The strike of the schistosity is about N.85°E. and the 
dip about 35°S. Near the surface the parting planes are stained with 
linionite. Pyrite crystals and small cavities, from which pyrite has been 
dissolved, are disseminated through the schist. Quartz stringers occur in 
some places. 

Below the outcrops of graphitic and other schist near the mouth of 
Gillespie Brook, no outcrops are found along Kettle River for a dis- 
tance of about three fourths of a mile. Beyond this, however, a 
group of exposures occurs in the southwestern part of section 28 
and the southeastern part of section 29, T. 46N., R. 20W., forming es- 
carpments along the river. The main escarpment on the east side of the 
river begins in the NWJ4 of SW'4 of section 28, about 300 feet east of 
the river, and continues with frequent exposures, southwest ward across 
the southeast corner of NE'4 of SE*4 of section 29, into the SEJ4 of 
SE/4 of section 29, where it forms a vertical cliff 20 feet high and about 
350 feet long on the south side of a big bend which occurs in the river 
at this place. The outcrop varies in width from a few feet at the north 
end to 400 feet at the south end. Exposures also occur on the west side 
of the river about 600 feet north of the big bend near the south line of 
the NE r 4 of SE l 4 of section 29. A rounded hill of rock about 230 feet 
wide east and west and about 260 feet long rises from the water's edge 
to a height of about 30 feet. 

The principal rock in this group of exposures is a dark greenish gray, 
banded, fine-grained, micaceous hornblende schist. There is an alterna- 
tion of bands of coarser and finer grained material. The schistosity is 
parallel to the banding or lamination. Stringers and lenses of quartz and 
quartz and calcite, varying from a fraction of an inch up to several 
inches in thickness in places lie parallel to the lamination. Generally in 
the larger lenses the quartz occupies the middle zone with calcite on 
either side. Quartz and calcite also form thin films along the joint planes. 
Pyrite occurs abundantly as small stringers along the banding and joint 
planes, and as crystals disseminated through the rock. At the north end 
of the escarpment the banding and schistosity strike about N./5°E. and 
dip 40°S., and at the south end they strike about N.85°E. and dip 2y°S. 
Gentle undulations occur between these points. At the outcrop on the 
west side of the river the strike of the banding and schistosity is about 
N.85°E. while the dip is to the south at low angles. 

In the northeastern part of the NWJ4 of NEJ4 of section 32, T. 46N., 
R. 20W., a low narrow outcrop lies at the water's edge on the west bank 
of Kettle River. The rock consists of alternate layers of coarse- and 
fine-grained, light gray phyllite. The surface is stained a rusty brown. 



ROCKS OF EAST CENTRAL MINNESOTA 



73 



The fine-grained layers are much contorted. Quartz lenses parallel to 
the lamination and schistosity are found in the fine-grained bands. The 
strike is about N.70°W. and the dip is to the southwest at a low angle. 

About a quarter of a mile below the exposure in section 32, Split Rock 
River flows into Kettle River from the northwest. A precipitous cliff of 
rock forms the southwest bank of Split Rock River from its mouth up 
stream for a distance of about 300 feet. Below the mouth, the cliff con- 
tinues southward along the west bank of Kettle River for about 1,200 feet. 
The rock is light to dark gray, fine-grained, micaceous phyllite. Alter- 
nations of fine-grained micaceous with coarser grained more siliceous 
layers bring out the original difference in the sediments. The bedding 
and schistosity are parallel and gently undulating. The finer grained 
layers are minutely crenulated. Small lenses of quartz occur locally 
parallel to the schistosity. A small pegmatite dike parallel to the schis- 
tosity is exposed for a few feet in a pit about 50 feet south of the mouth 
of Split Rock River. Several sets of intersecting joints cut the rock into 
polygonal blocks of various sizes. The average strike of the schistosity 
and lamination is about N.8o°E. and the dip varies from 15 to 35°S. 

A rock similar to that found at the mouth of Split Rock River is 
exposed over a large area in the center of the SE34 of section 32, T. 46N., 
R. 20W. It outcrops along the west bank of Kettle River for a distance 
of about a quarter of a mile and extends westward from the river about 
the same distance. The outcrops occur on a burnt-over tract as three 
large, flat, irregular mounds or ridges. The rock is light to dark gray, 
fine-grained, banded mica schist and phyllite, with marked schistosity in 
general parallel to the banding. The more micaceous or finer grained 
layers are crenulated. Quartz occurs in lenses parallel to the schistosity 
and also in veins from a few inches to several feet wide cutting across 
the schistosity. These quartz veins strike about N.io°E. and stand ver- 
tically. The strike of the bedding and schistosity is about east and west 
and the dip varies from 3 to 25 south, due to the broad low undulations. 

The phyllite and mica schist can be traced southward by frequent 
outcrops along the escarpment on the west side of the river to the south- 
east corner of section 32, T. 46N., R. 20W., where they form an over- 
hanging cliff about 20 feet above the water. This cliff continues south- 
eastward along the river for about 600 feet into Pine County, cutting 
across the northeast corner of section 5 and ending in the NW^l of 
NWJ4 of section 4, T. 45N., R. 20W. The average strike of the schis- 
tosity in this area is east and west and the dip is about 20°S. 

Farther south along Kettle River in northern Pine County, a number 
of scattered exposures of phyllite and mica schist occur extending through 
section 4 and thence southward as far as the south line of section 9, T. 



7A 



GEOLOGY OF EAST CENTRAL MINNESOTA 



45N., R. 20W., below which point no schist outcrops are known on Kettle 
River. In the southwestern part of the SEJ4 of NWJ4 of section 4, 
T. 45N., R. 20W., and extending southward into the NE# of SWJ4 of 
section 4, a long narrow outcrop of schist occurs on the east side of 
Kettle River. It forms a steep cliff along the river bank for about 400 
feet and then extends southeastward as a high escarpment, a short dis- 
tance from the river, for about 800 feet, with frequent outcrops. The 
rock consists of alternate layers of quartzitic and micaceous, light to 
dark gray phyllite and mica schist. The darker micaceous bands are 
crenulated, while the quartzitic layers are evenly schistose. Lenses of 
quartz, with scattered hornblende and garnet, occur parallel to the 
schistosity. Near the south end a vertical northwestward striking quartz 
vein about 10 feet wide cuts the schist. The strike of the layering of 
the mica schist is about east and west, and the dip varies from 2°N. to 
30°S., due to minor folding. 

About a quarter of a mile to the southwest, in the northeast corner 
of the SW34 of SW34 of section 4, T. 45N., R. 20W., are two small 
outcrops of mica schist on the west bank of Kettle River near the water's 
edge. The larger one is exposed for about 120 feet and rises from 2 
to 5 feet above the river. The smaller one is exposed for about 10 feet 
at the water's edge. The strike of the schistosity is about N.8o°E. and 
the dip is i8°S. 

In the E l / 2 of NWJ4 of section 9, T. 45N., R. 20W.. on the east bank 
of Kettle River, about 800 feet south of the Minneapolis, St. Paul, and 
Sault Ste. Marie Railroad bridge over Kettle River, an outcrop begins 
which continues southward along the river for about 650 feet. The out- 
crop lies at the water's edge. The rock is a light gray, garnetiferous, 
mica schist. Some layers are very siliceous and contain abundant small 
flakes of biotite and muscovite in a sugary quartz matrix. These alter- 
nate with layers in which the mica occurs in coarser folia and quartz is 
present in minor amount. Lense-like inclusions of sugary quartz con- 
taining abundant greenish black hornblende and some garnet are found 
parallel to the schistosity. The strike of the schistosity is about east 
and west and the dip varies from 20 to 30°S. 

A similar schist outcrops in the south central part of the NE% of 
SW>4 of section 9, T. 45N., R. 20W., on the southeast bank of Kettle 
River for a distance of about 600 feet. The strike is N.8o°E. and the 
dip is 30°S. This is the southernmost known outcrop of this series of 
rocks on Kettle River. About a mile and a quarter below it, sandstone 
blocks and boulders are abundant in the drift. With the exception of a 
small outcrop reported to occur near Willow River, however, there are 
no outcrops of the so-called Hinckley sandstone along Kettle River for 



ROCKS OF EAST CENTRAL MINNESOTA 



75 



more than 10 miles below the southernmost mica schist outcrop, and it 
is possible that the schist forms the bedrock over a part of this area. 

In the central and eastern parts of Carlton County, east of the Kettle 
River valley, outcrops of schist, phyllite, graywacke, and slate are more 
abundant than in the Kettle River region. They extend as scattered ex- 
posures and groups of exposures from Moose Lake in the south central 
part of the county to Cloquet in the northeastern part. In some places 
outcrops cover many acres while elsewhere small isolated exposures are 
found. For the most part, however, the mantle of glacial till is thin 
and the bed rock is probably not far below the surface throughout this 
region even where outcrops are small and scattered. 

At Moose Lake mica schist is found northeast, north, west, and south- 
west of the town along both the Minneapolis, St. Paul, and Sault Ste. 
Marie, and the Northern Pacific railroads. The scattered outcrops cover 
an area roughly 2 miles long and ranging up to perhaps 1,000 feet in 
width. In the NE% of section 20, T. 46N., R. 19W., the Minneapolis, 
St. Paul, and Sault Ste. Marie Railroad runs in a rock cut from a few 
feet up to 15 feet deep for a distance of about 2,000 feet. The rock is 
light to dark gray, finely to coarsely crystalline phyllite and mica schist. 
The lighter gray layers are siliceous and fine-grained, while the darker 
layers are rich in mica and are more coarsely crystalline. The micaceous 
layers show fine crenulation and give shining, glossy, cleavage surfaces. 
Quartz occurs in lenses parallel to the schistosity and also as veins and 
thin films along some of the joint planes. About 800 feet from the north 
end of the cut, a vertical dike of fresh, grayish black, fine-grained, 
massive, igneous rock about 11 feet wide cuts the schist, striking about 
N.75°W. At the contact with the schist, the rock is extremely fine tex- 
tured but becomes coarser toward the center of the dike. The contact is 
sharp and the schist is hardened within a few inches of the contact and 
its schistose structure is destroyed. The schistosity shows low, broad, 
undulations along the walls of the cut. The average strike is about 
N.6o°E. and the dip varies from 5 to 40°SE. 

The phyllite and mica schists continue as a ridge southwestward from 
the railroad cut for about a mile, where they are cut through by the 
Northern Pacific Railroad in the NE*4 of NW}4 of section 29. The 
ridge lies between the tracks of the two railroad lines, and rises about 30 
feet above the grade of the Northern Pacific Railroad. Small outcrops 
are abundant along it. To the southwest it is lower and gradually dis- 
appears beyond the Northern Pacific railroad tracks. In the rock cut 
on the Northern Pacific Railroad in the southeast corner of S~E% of 
SW% of section 20, and in the northeastern part of the NE*4 of NW T 4 
of section 29, the rock is exposed continuously for about 500 feet on the 



76 



GEOLOGY OF EAST CENTRAL MINNESOTA 



northwest side of the railroad. It shows low, broad, undulations in the 
schistosity which has a general low dip to the east. 

Just northeast of Moose Lake in the north central part of the NE}4 
of NW^4 of section 21, T. 46N., R. 19W., is a low, rounded mound 50 
by 60 feet, about 170 feet northwest of the Northern Pacific Railroad 
tracks. The mound consists of interlayered quartzitic and micaceous 
schists. The light gray siliceous layers are thicker than those in the 
rock forming the ridge northwest and west of Moose Lake; individual 
layers reach a thickness of 2 feet. The strike of the layering is about 
N.85°E. and the dip i$°S. 

About one third of a mile farther northeast, an outcrop of mica schist 
occurs in the bed of a small stream in the north central part of the SWJ4 
of SE T /4 of section 16, T. 46N., R. 19W. The rock is exposed for about 
40 feet and is similar to that in the cut at Moose Lake. The strike of 
the layers is about N.42°E. and the dip is 6°SE. Loose boulders and 
irregular blocks of the same kind of rock occur abundantly in the drift 
for several hundred feet to the southwest. 

The next important group of outcrops to the northeast is that near 
Barnum. At the railroad station and extending westward from it is a 
large flat area of irregular outline in which rock exposures are abundant. 
The area is in the western part of the NW*4 of section 1, T. 46N., R. 
19W., the NE*4 and the northeastern part of the NE*4 of NW>4 of 
section 2, T. 46N., R. 19W., the southeastern part of the SK%. of SW^4 
and the southern part of the SE*4 of section 35, T. 47N., R. 19W. It 
rises as a small upland 3 to 10 feet above the surrounding low land, and 
in places forms a vertical escarpment along the borders. Parts of it 
are concealed by a thin veneer of soil. The rock is phyllite and mica 
schist. Thin layers of glossy, dark gray, micaceous rock with wavy 
cleavage planes are interbedded with heavy, massive, layers of fine- 
grained lighter gray graywacke. These massive beds are from 2 to 10 
feet thick and some of them are highly calcareous. Scattered stringers 
and lenses of quartz parallel to the schistosity are abundant in the mica- 
ceous layers, and calcareous concretions or lenticular rusty brown cavi- 
ties from which they have been dissolved, are found in some of the mas- 
sive beds. The average strike of the bedding is about east and west and 
the dip varies from o° to 35°S. Small drag folds pitching from o° to 
35 to the east are abundant in the micaceous layers. The outcrop as a 
whole appears to be on the south limb of a larger eastward pitching anti- 
cline. 

About 5 miles west of north from Barnum on the West Branch of 
Moose Horn River, in the SW% of NW>4 and the NWJ4 of SWJ4 of 
section 15, T. 47N., R. 19W., black slate and associated altered diabase 



ROCKS OF EAST CENTRAL MINNESOTA 



77 



crop out along the river bed for a distance of about one half mile. The 
exposures are practically continuous for this distance and rise on either 
side of the stream from a few inches up to about 5 feet. The southern 
part of the outcrop is black carbonaceous slate, probably largely graphitic, 
the strike and dip of which vary in short distances due to minor folding. 
Large crystals of pyrite occur in it abundantly. A well-developed fracture 
cleavage has been superimposed on the slaty cleavage and is almost at 
right angles to it. The large structural feature indicated by the slate out- 
crop is a southward dipping monocline, with minor drag folds, forming 
the south limb of an eastward pitching anticline. The northern part of 
the outcrop consists of dark, grayish-green, medium-grained, altered dia- 
base, containing abundant scattered pyrite. The contact of the diabase 
and slate is not exposed, but it seems probable that the diabase forms a 
large intrusion in the black slate. 

About 6 miles northeast of Barnum near Mahtowa, a station on the 
Northern Pacific Railroad, are numerous exposures of schist and slate 
and some of dark igneous rock. They are found chiefly along Moose 
Horn River and its branches from Mahtowa northward to Park Lake. 
Throughout most of the region the bedrock where not exposed is com- 
paratively near the surface. 

In the of SE>4 of section 8, T. 47N., R. 18W., in a meadow 

about 1,000 feet southeast of the center of the section, two pits, 100 feet 
apart, pass through a thin mantle of glacial drift into bed rock. The 
pits are 8 by 12 feet and are reported to be 30 feet deep. The rock con- 
sists of layers of a light gray and dark gray, fine-grained phyllite and 
slate, black carbonaceous slate, and grayish black, dense, finely laminated 
amphibole schist or amphibolite. Small quartz lenses and stringers of 
carbonate occur parallel to the cleavage. Crystals of pyrite are abundant. 
In the west pit the cleavage and lamination strike about N.77°W. and 
dip i6°S., and in the east pit they strike about N.70°W. and dip 85°N. 
About 700 feet northeast of these pits is another pit, partly filled, which 
has black carbonaceous slate on the dump. 

In the northwestern corner of this 40-acre tract are several small, low, 
shattered outcrops of a light gray, fine-grained phyllite. No satisfactory 
observations could be made as to strike and dip. 

Another outcrop of phyllite, slate, and amphibolite occurs in the north- 
eastern part of section 8 in a low round knoll on the north-south line 
between the NE*4 of NE}4 and NW% of NE}4, about 80 feet south of 
the north line of the section. The knoll is 250 feet wide east and west 
and about 300 feet long, lying west of Moose Horn River. The outcrop 
is mainly fine-grained laminated amphibole schist, but includes some layers 
of finely micaceous phyllite and slate. The cleavage is well developed in 



GEOLOGY OF EAST CENTRAL MINNESOTA 



the micaceous phases, but less so in the amphibolitic phases. Some mica- 
ceous layers show crenulated cleavage. The weathered amphibole schist, 
or amphibolite, is speckled with abundant minute spots of light brown 
ocherous limonite, the decomposition product either of ferrous carbonate 
or of some ferrous silicate. Some of the rock has a rusty brown color 
due to the abundance of limonite. The slate and phyllite layers are sub- 
ordinate to the amphibolite layers. All are comparatively thin-bedded. 
The rock lies in a series of small anticlines and synclines forming a 
larger westward pitching anticline. The axis strikes about N.75°E. and 
pitches about 6o°W. 

About 250 feet southwest of the northeast corner of the SWJ4 of 
SE34 of section 5, T. 47N., R. 18W., two pits were sunk about ten 
years ago in black carbonaceous slate, probably largely graphitic. The 
pits are 10 by 15 feet, and about 30 feet deep. The north pit passes 
through 20 feet of black slate into a glossy, light gray, micaceous slate 
and phyllite. The strike of the schistosity of the black slate is N.70°E., 
and the dip is 26°S. The south pit was sunk on a quartz vein about 2 l / 2 
feet wide. The vein strikes about north and south and stands vertically, 
while the schistosity of the slate strikes N.66°E. and dips 37°S. Pyrite 
occurs abundantly in the black slate as narrow stringers parallel to the 
schistosity and also as large cubical crystals, an inch or more along the 
side, which have grown without reference to the cleavage. In many 
cases the cleavage of the slate is shown by parallel lines cutting across 
the crystals of pyrite, and thin parallel folia of slate are frequently in- 
cluded within the pyrite crystals. This shows that the pyrite is second- 
ary, having developed during or after the metamorphism of the black 
slate in the same manner that the staurolite crystals are developed in 
the garnetiferous schist of the Little Falls region. (See page 62.) 

A small, low outcrop of black slate is found 450 feet south of the 
pits in the same 40-acre tract. 

In the E T / 2 of SEJ4 of section 5, T. 47N., R. 18W., small exposures 
of black slate occur frequently on both sides of the northwest road and 
in the bed of Park Lake River. These outcrops are low and shattered. 
A small pit on the east side of the road, about 800 feet north and 300 
feet west of the southeast corner of the section, was dug in black slate, 
as shown by the material on the dump. Much pyrite occurs in this slate 
along cleavage planes. About 50 feet east of the pit is a shallow trench, 
2 feet wide, trending north and south, exposing the black slate for a 
distance of 60 feet. The rock is covered by only a few inches of soil. 
The cleavage strikes N.70°E. and dips 75°S. About 200 feet north of 
the pit on the west side of the road black slate crops out over a small area, 
while 400 feet farther northeast on the east side of the road, light gray 



ROCKS OF EAST CENTRAL MINNESOTA 



79 



micaceous quartzite or graywacke crops out along the east section line 
and continues into section 4. The rock is massive and in general has a 
poorly developed cleavage. The strike of the layering is N.70°E. and 
the dip is 50°S. Several small exposures of black slate occur a short 
distance north and northwest of this outcrop on both sides of the road. 

In the SWM of NEJ4 of section 5, T. 47N., R. 18W., farther north- 
westward along the road, black slate is exposed at frequent intervals for 
a distance of about 700 feet. The rock occurs in low ridges on both sides 
of the road and has also been exposed in the grading. The strike of the 
cleavage at the south end of this area is about N.75°E. and the dip is 
40°S. ; at the north end the strike is about N.75°E. and the dip is 75°S. 

A number of outcrops of graywacke, slate, and phyllite occur along 
the west line of section 4, T. 47N., R. 18W. south of the west quarter 
corner. As has been mentioned, some of these extend westward into sec- 
tion 5 and most of them also continue for a short distance eastward into 
section 4. 

An outcrop of light gray phyllite and black slate occurs near the 
center of section 4. It lies on the west side of the north-south road 
running along the east line of the NE*4 of SW34 of the section. The 
outcrop begins at the northeast corner of this 40-acre tract (at the 
center of section 4) and continues southward along the road for about 
800 feet. The outcrop extends westward from the road 50 feet at the 
north end of the outcrop, and 200 feet at the south end. It is a low flat 
exposure, the west margin forming a westward facing escarpment 6 to 
8 feet above the general level of Moose River, which is a short distance 
west. The outcrop is made up of alternate layers of light to dark gray 
phyllite and black slate. The dark bands contain carbonate minerals and 
are mottled with small porous specks of limonite. The outcrop is a large 
anticline with small drag folds on both limbs. The strike of the axis is 
N.8o°E. and the pitch is 20°E. 

A number of scattered outcrops of light gray phyllite and dark gray 
slate occur in the E*/ 2 of NEJ4 of section 4, T. 47N., R. 18W. The 
rock is badly broken and rises only a few inches above the soil. No sat- 
isfactory observations could be made on the attitude of the rock in most 
of the outcrops. Near the southeast corner of the NE}4 of NE^ of 
the section, however, dark gray slate striking about N.86°E. and dipping 
45 °S. crops out in a low ridge about 200 feet long, east and west and 
about 60 feet wide. 

Three and one-half miles north of east from Mahtowa near the 
center of NE}4 of SEJ4 of section 1, T. 47N., R. 18W., is a small out- 
crop of light gray, fine-grained graywacke. The outcrop is broken and 
covered with loose blocks. No satisfactory observations could be made 
as to strike and dip. 



Bo 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Several rock exposures of varying size are found in the SE% of 
section 31, T. 48N., R. 18W. The northern and easternmost of these 
is a low outcrop of a light gray, medium-grained graywacke occurring 
near the center of the east-west line along the north side of the NE}4 of 
of the section. The attitude of the graywacke is uncertain. 

In the W^2 of the quarter section, several outcrops are found con- 
sisting mainly of slate, phyllite, and graywacke. The most northerly of 
these is about 400 feet west and 350 feet south of the northeast corner 
of the NW74 of SEJ4 of section 31. It is 5 feet wide and 10 feet long 
and rises only a few inches above the surface. The rock is dense, light 
gray, fine-grained graywacke. The weathered rock is full of minute 
brownish yellow specks of limonite. The graywacke has a poorly de- 
veloped schistosity, which strikes N.84°E. and dips 64°S. 

About 400 feet southwest of this outcrop, a 3-inch vertical hole was 
drilled to a depth stated to be 330 feet. No accurate log was kept of 
the drilling, as far as known. The information available, however, and 
an examination of the core, indicates that the upper part of the hole is 
in dense, dark gray, finely micaceous graywacke and that below this, 
black, carbonaceous (probably largely graphitic) slate was encountered. 
The cleavage in both rocks is well developed. The cleavage surface of 
some of the black slate has a mottled appearance caused by small, scat- 
tered, shiny, black particles. The dip of the cleavage is 50 , probably to 
the south. 

About 450 feet somewhat east of south from the drill hole there are 
several small outcrops of light gray, medium-coarse graywacke. In some 
there is a slight orientation of the flaky minerals producing a poorly 
developed schistosity. Similar rock in small scattered exposures is found 
about 1,000 feet west of this exposure in the southeastern part of the 
NEJ4 of SWJ4 of section 31. 

In the western part of the SE J 4 of SEJ4 of section 31, T. 48N., R. 
18W., about one third of a mile southeast of the drill hole, are several 
long, low, narrow, parallel ridges consisting of alternate layers of phyl- 
lite, slate, and graywacke. The ridges vary in length from 50 to 300 
feet and in width from 10 to 50 feet, their longer axes being parallel to 
the strike of the rocks. The slate and phyllite are dark gray and occur 
as narrow beds from a few inches to a foot thick in the heavy -bedded, 
lighter gray, fine-grained graywacke. These rocks lithologically are sim- 
ilar to those south of Park Lake, which are described later. The strike of 
the bedding is N.53°E. and the dip is 42°SE. Minor drag folds in the 
slate pitch northeastward at low angles. The rocks appear to be on the 
south limb of a larger anticline. 

About ten years ago, in prospecting for coal, two pits were sunk just 
west of the dwelling house of J. P. Peterson in the SW>4 of of 



ROCKS OF EAST CENTRAL MINNESOTA 81 

section 32, T. 48N., R. 18W. The larger pit is 350 feet south and 70 
feet east of the northwest corner of this 40-acre tract, on the east 
side of Park Lake Creek. It is 15 feet by 20 feet, and 30 feet deep. In 
the bottom a 3-inch vertical hole was put down 80 feet. The pit is now 
caved and partly filled with water. The small pit, 75 feet to the south, 
is 5 feet by 5 feet and was sunk to a depth of 8 feet. The surface ma- 
terial is from 2 feet to 10 feet in thickness. The rock in both pits is 
black carbonaceous slate, much fractured and impregnated with pyrite 
and quartz. The more highly carbonaceous phases of the slate are dull, 
earthy, and soft. Other phases are harder and show slickensides in sev- 
eral directions. Some graphite in shiny films is developed along the 
slickensides and along fracture planes. On account of the caving and 
water, the attitude of the slate could not be determined. The cleavage in 
a piece of core taken from the larger pit shows a dip of about 45 . This 
dip is probably south, judging from the structure of slate outcrops 
nearby. 

In the region south and southwest of Park Lake are extensive out- 
crops of slate and graywacke, covering considerable areas in sections 
29, 30, and 31, T. 48N., R. 18W. The northernmost of these exposures 
are found on the south shore of the lake in the SE^J of NW}4 of sec- 
tion 29. They rise about 10 feet above the level of the lake and slope 
out into the lake beneath the water. Near the shore they form several 
small, low rock islands. Most of the rock is massive, medium-grained, 
light gray graywacke, with small bands of fine-grained, darker, slaty ma- 
terial, with well-developed cleavage. Glacial grooves and striations trend- 
ing N.6o°W. are seen on outcrops of the coarser phases. The strike of 
the formation varies from east and west to N.7o°W. and the dip varies 
due to minor folding. Drag folds show that the outcrops are on the north 
limb of an anticline. 

About 350 feet south of the center of section 29, T. 48N., R. 18W., 
on the north-south road running southward from Park Lake, graywacke 
and slate exposures occur as low, narrow ridges crossing the road in an 
east-west direction. The width of the group of exposures where it 
crosses the road is more than a quarter of a mile, and it extends east- 
ward into the of SE}4 of section 29 for a maximum distance of 
250 feet. The group is at the northeast end of a large area of exposures 
which continues from this point southwestward across the SW% of sec- 
tion 29, the southeast half of the SE T 4 of SE^ of section 30, the north- 
west half of NE54 of NE^ of section 31, T. 48N., R. 18W. Through- 
out this area exposures are more or less continuous. South of its west 
end, small scattered rock ridges occur for a distance of about 800 feet 
farther, being partly in the SE% of NE^ and partly in the SW% of 



82 



GEOLOGY OF EAST CENTRAL MINNESOTA 



of section 31. The main belt of exposures consists of many parallel 
rock ridges of varying size. They range from 5 feet to 600 feet in width 
and from 10 to 2,100 feet in length, and rise above the surrounding soil 
from a few inches up to 15 feet. The ridges are separated from each 
other by lower marshy ground, the distance between the ridges varying 
from 20 feet to 400 feet. Their longer axes strike east-west or a few 
degrees south of east. 

The outcrops throughout the area are made up mainly of alternate 
bands of medium coarse-grained graywacke and fine-grained slate. These 
bands vary in thickness from a fraction of an inch up to several feet ; the 
graywacke is more abundant than the slate and occurs in thicker, more 
massive beds. Locally fine micaceous phyllite occurs where deformation 
has been somewhat more intense, or where shearing has taken place. 
The slate is darker than the graywacke and has a well-developed cleav- 
age. Some of the coarser graywacke phases, also, show a slight schistos- 
ity. Small, elongated, nodules or concretions of calcareous material oc- 
cur in broken bands in the graywacke, and appear as cavities on the 
weathered surface. Quartz occurs sparingly as stringers between the 
bedding in the minor folds and as thin films along some of the joint planes. 
Small grains of pyrite are abundant in the coarser material, and on 
weathering leave small cavities with a rusty stain. Several sets of joints 
cut the rocks into polygonal blocks of various sizes. 

The rocks are folded into a series of small anticlines and synclines of 
several orders, with the axes at the crests and troughs horizontal or 
pitching up to 5 either to the east or to the west. The average strike 
of the axial planes is about N.77°W. From a study of the drag folds 
and the relation of cleavage to the bedding, it appears that the Park Lake 
series of outcrops forms a low flat anticlinorium. 

A short distance south of the main Park Lake area of slate and gray- 
wacke outcrops is a small exposure of phyllite. It is located on the north 
side of the east-west road between sections 29 and 32, T. 48N., R. 18W., 
about 100 feet east of the southwest corner of the SE*4 of SW}4 of 
section 20, T. 48N., R. 18W. The rock is light gray, and on the weathered 
surface is mottled, the mottling being due to small cavities containing 
limonite derived from the weathering of some disseminated iron-bearing 
mineral. The schistosity strikes N.82°W. and dips 35°S. 

Numerous outcrops of slate and graywacke are found in the region 
between Park Lake and Atkinson, most of them are ridges trending 
approximately east and west. The most westerly of these is about a mile 
south of east from Park Lake in the southeastern part of section 28, T. 
48N., R. 18W. Two long, low, narrow ridges are situated along the 
east-west road on the south line of the of SE% of section 28. 



ROCKS OF EAST CENTRAL MINNESOTA 



83 



The south ridge, about 30 feet wide, lies about 200 feet west of the south- 
east corner of the 40-acre tract, and extends westward along the road 
for about 470 feet. Ten feet north on the roadside is a smaller ridge 
about 200 feet long and 25 feet wide. The outcrop in the road consists 
of gray, medium fine-grained graywacke cut into polygonal blocks, 1 to 
4 feet long by intersecting joints. Cavities formed from the more easily 
weathered carbonate nodules occur in bands bearing about east and west. 
Locally there is a poorly developed schistosity striking N.8o°W. and 
dipping 34 S. The outcrop north of the road is made up of alternate 
bands of fine-grained, dark gray slate and medium fine-grained, lighter 
colored graywacke. These bands vary in thickness from 2 inches to a 
foot or more. Single bands can .be followed the entire length of the 
outcrop. The strike of the layers is N.88°W. and the dip 56°N. 

About a mile east of the exposures in section 28, a series of more or 
less parallel east-west rock ridges occurs in the east central part of the 
Ey 2 of SE}4 of section 27, T. 48N., R. 18W., and continues eastward 
across the central part of the SW^4 of section 26. The ridges vary up 
to 600 feet in length and from 10 to 300 feet in width, and rise from 
2 to 20 feet above the intervening depressions. Prominent ridges are 
found on both sides of the east- west road leading to Atkinson, the largest 
being on the south side. The rock is slate and fine-grained graywacke 
occurring in alternate beds. The beds are from a fraction of an inch 
to 10 feet or more thick. The slate beds have well-developed cleavage 
and are darker than the graywacke. Some layers are calcareous. Much 
of the rock is finely mottled for a few inches below the weathered sur- 
face, due to an abundance of small specks of limonite. Carbonate con- 
cretions or nodules, or cavities from which they have been weathered 
out, are found in some of the graywacke bands. Spongy quartz with 
soft earthy limonite in the cavities, occurs as veins along some of the 
joint planes and also parallel to the bedding. The limonite is a weather- 
ing product of siderite and perhaps of pyrite. The average strike of the 
bedding is about east and west, and the dip varies from 30 to 70°N. due 
to drag folds. The strike of the cleavage is about east and west and the 
dip is about 70°S. These rock ridges are structurally on the north limb 
of a larger anticline. 

The next group of outcrops to the east is west, north, and east of 
Atkinson. It extends from the east central part of section 26, T. 48N., 
R. 18W., eastward through the central part of section 25, the last out- 
crops to the east being in the east central part of section 25. The ex- 
posures form a belt of more or less parallel east-west trending ridges 
occupying an area about a mile long east and west and about one fourth 
of a mile wide. A few outlying exposures are found west and north 



GEOLOGY OF EAST CENTRAL MINNESOTA 



of the main belt. The individual outcrops vary from mere knobs to 
ridges 600 feet long and 300 feet wide. Several of the ridges west of 
Atkinson are 10 to 30 feet high. The rock consists of alternate layers of 
light to dark gray graywacke, and dark gray to black slate. The beds 
vary in thickness from a few inches to several feet. The weathered 
phases are mottled with fine specks of limonite. Quartz veins and quartz 
siderite lenses are common along the axes of the small folds and pinch 
out along the limbs. The siderite locally has been entirely altered, and 
the lenses consist of a spongy mass of quartz and earthy limonite. Sev- 
eral pits have been sunk in prospecting for gold in different parts of the 
area where these lenses are abundant. In two pits in the northeast cor- 
ner of the NW r 4 of SW r 4 of section 25, a small brecciated zone is ex- 
posed in the dark-colored slate. Quartz, siderite, pyrite, and some cal- 
cite cement the breccia and extend into the wall rock for several feet as 
stringers between the laminae of the slate. The rocks in the central part 
of the area lie in a series of folds, with smaller folds down to minute 
plications superimposed on their limbs. The axes of the larger folds 
strike about east and west, and are either horizontal or pitch at a low 
angle to the east. At the western end of the main belt of outcrops in 
the northeast corner of the NEJ4 of SE*4 of section 26, there is a low, 
flat exposure about 250 feet east and west and 100 feet wide. The 
average strike of the beds at this place is about N./5°E. and the dip varies 
due to minor folding. The axial lines of the minor folds strike about 
N.85°E. and pitch 8°E. The rock is light gray graywacke, some of which 
shows well-developed cleavage. Locally tiny flattened particles of dark 
gray slate are embedded in the lighter colored graywacke. Another small # 
outcrop in the western part of the main belt of exposures occurs on the 
northwest-southeast road in the southern part of the SW^J of of 
section 25. The strike of the beds is about N.85°E. and the dip is 8o°N. 

Outlying exposures of graywacke and slate occurring northwest of 
the main belt of outcrops form two long, low, narrow, parallel ridges in 
the SEJ4 of NE T 4 of section 26, and the SW^ of NWJ4 of section 25. 
The ridges extend east and west for nearly a quarter of a mile and vary 
from 20 to 80 feet in width. They rise only a few feet above the sur- 
rounding flat area. The rock is mainly light gray, mottled graywacke 
with a few thin interbedded layers of slate. The strike is about N.85°E. 
and the dip varies from 40 to 6o°S. 

An outlying exposure north of the main belt of outcrops at Atkinson 
occurs in the north central part of the SWJ4 of NE^J of section 25, 
forming two low, flat, parallel, east-west ridges. They are about 250 
feet long and 50 feet wide. The rock is light gray, siliceous graywacke 
in heavy layers with thin interbedded layers of dark gray slate. The 
beds strike about east and west and dip 45°S. 



ROCKS OF EAST CENTRAL MINNESOTA 



85 



About half a mile east of Atkinson on Black Hoof River, is the east- 
ernmost outcrop of graywacke and slate in this vicinity. It is located 
in the west central part of the NE^4 of SW>4 of section 30, T. 48N., 
R. 17W., and consists of several low knobs of fine-grained, light gray 
graywacke. The outcrop rises near the north-south road on the west 
side of this 40-acre tract and extends eastward for about 650 feet. 
Knobs occur on both sides of the river rising 8 to 10 feet above the water 
and sloping gently toward the east. The rock is in places schistose and 
is cut into irregular blocks by several sets of joints. Concretions or 
nodules of carbonate minerals occur sparingly. The beds strike east 
and west and dip south at a low angle. 

The belt of exposures extending from Park Lake eastward through 
sections 28, 27, 26, and 25, T. 48N., R. 17W., which is described above, 
includes the principal rock outcrops of this region. A more or less par- 
allel belt composed of fewer and more scattered exposures, occurs south 
of this larger belt, extending in an approximately eastward direction from 
the eastern part of section 34, T. 48N., R. 18W. through sections 35 and 
36, T. 48N., R. 18W., into section 31, T. 48N., R. 17W. The exposures 
of the northern belt have structures indicating that in general they are 
situated on the north limb of a large anticline, those in the southern belt 
appear to be either near the crest or on the south limb of an anticline. 

The westernmost of the exposures on the southern belt consists of a 
large irregular knoll of rock located in the east central part of the of 
NE}4 of section 34, T. 48N., R. 18W., and extending for about 200 feet 
into the southwest corner of the NW>4 of NW^ of section 35. The 
knoll is about 1,000 feet long east and west and about 800 feet wide north 
and south and rises about 60 feet above the surrounding country. It is 
made up of thick beds of a light gray, fine-grained graywacke separated 
by thin layers which are more slaty and darker in color. The slaty layers 
are micaceous and have well-developed schistosity. Locally they are 
crenulated and contain lenses of quartz parallel to the foliation. 
Intersecting sets of joints cut the rock into large irregular blocks, and 
some of the joints are filled with quartz. Lenticular depressions, stained 
black or rusty brown, from which nodules of carbonate minerals have 
been dissolved, occur parallel to the strike of the bedding in some of the 
coarser layers. The outcrop appears to represent an eastward pitching 
anticline, the strike of the axial line being about N.85°E. and the pitch 
io°E. Small drag folds occur on both limbs of the anticline. 

In the northeastern part of the SW>4 of NE^ of section 35, T. 48N., 
R. 18W., just south of the north line and about 100 feet west of the 
Northern Pacific tracks, is a low glaciated ridge of phyllite. The ridge 
is about 150 feet long east and west and 75 feet wide, and rises a few 



So 



GEOLOGY OF EAST CENTRAL MINNESOTA 



feet above the surrounding meadow. Thick, light gray, fine-grained, 
arenaceous layers alternate with dark gray, slaty layers. Some of the 
slaty layers show a considerable development of fine mica along cleavage 
planes. Concretions of carbonate minerals are abundant in some of the 
layers. At the east end of the outcrop they are more resistant than the 
enclosing rock and stand out as tiny knots, while at the west end they 
have been dissolved, leaving rusty brown or black pod-like depressions. 
Several sets of intersecting joints cut the rock into large irregular blocks. 
The strike of the schistosity is about N.85°E. and the dip is 45°S. The 
bedding strikes N.82°W. and the dip varies in short distances, due to 
minor folding. This outcrop is on the south limb of a larger anticline. 

About a mile northeast of the exposure in section 35, near the north 
quarter corner of section 36, T. 48N., R. 18W., rock is exposed in sev- 
eral small areas. It is mainly light gray, fine-grained graywacke, much 
weathered and shattered. The beds appear to He almost flat. 

The easternmost rock exposures of the southern belt of outcrops oc- 
cur in the SWJ4 of section 31, T. 48N., R. 17W., along Black Hoof River 
and west of it, where there are three or four separate outcrops of gray- 
wacke and slate. An exposure of graywacke and slate in a low flat ridge 
is found in the northeastern part of the SWJ4 of SW}4 of the section. 
It lies on a northward facing slope and rises about 10 feet above a swamp 
to the north of it. The ridge is 600 feet long east and west and 300 feet 
wide. Small, shattered exposures lie to the east, south and west of the 
main outcrop for several hundred feet. In the main exposure, layers of 
light gray, fine-grained graywacke, hard and siliceous, and with poorly 
developed cleavage, alternate with darker slaty layers having well- 
developed cleavage. The layers vary from a few inches to several feet 
thick, the graywacke beds being thicker. The slaty layers show fine mica 
developed along cleavage planes. Lenses and stringers of quartz occur 
parallel to the bedding. The rock lies on the crest of a large anticline 
with small anticlines and synclines on either limb. The axial line of the 
anticline strikes about east and pitches about io°E. 

About a quarter of a mile to the north of this exposure, in the NWJ4 
of SWJ4 of the section, is a ridge of dark gray slate trending northwest. 
The ridge rises only a few feet above the surrounding flat, swampy coun- 
try. It is about 500 feet long and about 200 feet wide. The rock is 
uniform, dark gray slate with fine mica developed locally along cleavage 
planes. Thin laminae and specks of limonite lie parallel to the cleavage 
and also occur as small spots throughout the rock. In the fresher phases 
pyrite is abundant, indicating that the limonite is derived from pyrite 
by weathering. The slate forms a synclinal trough that pitches a little 
south of east at a low angle. 



PLATE IX 




B. OUTCROPS OF SLATE AT THE NORTHERN PACIFIC RAILROAD BRIDGE OVER ST. LOUIS 
RIVER BELOW THOMSON DAM, CARLTON COUNTY. LOOKING SOUTH WESTWARD 



ROCKS OF EAST CENTRAL MINNESOTA 



87 



West of these two ridges in the western part of the SWJ4 of section 
31, and extending into the SE^ of section 36, T. 48N., R. 18W., are 
small, low, badly broken exposures of dark gray slate. No outcrop show- 
ing the attitude of the rock was found. 

About a quarter of a mile east of the outcrops described above in 
the NW34 of SE}4 of section 31, several small exposures of light gray, 
medium-grained graywacke are found along Black Hoof River. The 
rock is hard and siliceous and shows no cleavage. 

The next group of exposures northeast of the Atkinson-Park Lake 
region are those in the vicinity of Otter Creek in sections 16, 17, and 20, 
T. 48N., R. 17W. Most of these exposures occur along the Northern 
Pacific Railway which runs northeastward from Atkinson to Carlton. 
Some are found along Otter Creek. ' 

A short distance northeast of the center of the SW% of NW^4 of 
section 20, T. 48N., R. 17W. are two small low ridges of slate and gray- 
wacke. The outcrop begins along the northeast-southwest road which 
runs parallel to the Northern Pacific railway tracks, northeastward from 
Atkinson, and continues southeastward across the Northern Pacific tracks. 
It is about 300 feet long and about 46 feet wide. Thin bands of dark 
gray slate are interlayered with thick beds of lighter gray graywacke. 
The rock is broken into large blocks by intersecting joints. Quartz, with 
small crystals of pyrite, occurs in narrow veins and lenses. The bed- 
ding strikes about N.85°W. and dips 47°N. 

About a fourth of a mile northeast of this outcrop in the east cen- 
tral part of the NE*4 of NW% of section 20, several low east-west rock 
ridges occur along the Northern Pacific tracks and along the highway. 
They vary in length from 50 to 300 feet and in width from 30 to 100 
feet. The rock consists of dark gray slate, some layers of which are 
very siliceous and have poorly developed cleavage, while other layers are 
less siliceous and show good cleavage. The layers range in thickness 
from a fraction of an inch to several inches. Small crystals of calcite 
and siderite are abundant in the more siliceous layers. In one layer 
small flattened oval particles of black slate are embedded in the lighter- 
colored siliceous slate. The rocks lie in a series of small anticlines and 
synclines, the axial lines of which strike about N.8o°E. and pitch from 
io° to 20°E. 

About 200 feet southwest of the Northern Pacific Railway bridge 
over Otter Creek, in the northeastern part of the SE% of SE54 of section 
17, T. 48N., R. 17W., the railway passes through a rock cut for about 
no feet. This cut is through the northwest corner of a high east-west 
rock ridge about 500 feet long and 175 feet wide which rises about 50 
feet above Otter Creek. Layers of slate and schistose graywacke are 



n 



GEOLOGY OF EAST CENTRAL MINNESOTA 



exposed along the cut, striking N.8o°E. and dipping 30°N. The fol- 
lowing succession of beds is found. At the bottom, with 5 feet exposed, 
is a dark gray, fine-grained, siliceous slate. On the cleavage surfaces 
are seen a few rounded, flat, black particles, probably squeezed slate frag- 
ments. Specks of pyrite and some carbonate are disseminated through 
the rock. Above this layer are 3 feet of uniform, dull, black, dense slate ; 
above that 4 feet of dark gray siliceous slate and schistose graywacke 
which in places contain pebbles of fine-grained, darker slate, flattened 
transversely to the cleavage. The pebbles range from a fraction of an 
inch to several inches in the longer diameter. Alternating thin layers 
of slate of different coarseness of grain and color are clearly visible in 
this bed, the coarser layers carrying the slate pebbles. The cleavage cuts 
transversely across the layers and has fine flakes of mica developed along 
it. The coarser layers are finely mottled, due to abundant limonite 
specks, probably derived from the alteration of siderite. The next 5 feet 
consist of dark greenish phyllite finely mottled with tiny specks of limo- 
nite. A few small oval flattened particles of dark slaty material are 
found on the cleavage faces. Above this bed is a layer of dark gray, 
coarse-grained, schistose graywacke 3 feet thick. It is similar to the 
bed below it, but coarser, containing abundant small rounded particles of 
dark slaty material, as well as numerous grains of quartz. Small specks 
of limonite and of white carbonate minerals are scattered through the 
rock. Pyrite is found locally and fine mica is developed along cleavage 
planes. The cleavage is well developed. The top layer in the cut is a 
dark gray to black, uniform, fine-grained slate. The thickness could not 
be determined, but about 15 feet are exposed in the cut. The cleavage 
strikes about N.87°E. and dips 75°S. 

Northeastward from Otter Creek to Carlton a few ridges of slate 
and graywacke are found both north and south of the Northern Pacific 
tracks. The ridges trend about east and west and rise from a few feet 
to 50 feet or more above the surrounding flat region. These ridges have 
not as yet been examined in detail. To the northeast they connect with 
the extensive exposures of rock along St. Louis River in the vicinity of 
Carlton. Thomson, and Cloquet. 

In the St. Louis River valley slates and graywacke with a few dikes 
of dark igneous rock are exposed almost continuously along the river 
from the center of section 1, T. 48N., R. 16W. on the southeast, to the 
eastern part of section 15, T. 49N., R. 17W. on the northwest, a distance 
by river of about 15 miles. These rocks rise as high steep escarpments 
on both sides of the river for much of the distance and within this gorge 
are numerous falls and rapids. They also rise as long, narrow, east and 
west rock ridges, that extend from a few hundred feet to several miles 
on either side of the river gorge. These ridges are very numerous 



PLATE X 




B. SMALL DIKE OF DIABASE INTRUDING SLATE AT CARLTON 



ROCKS OF EAST CENTRAL MINNESOTA 



89 



locally. In the region around Carlton and Thomson they cover several 
square miles and continue southwestward from Carlton for some distance 
in the direction of Otter Creek. 

In the southeastern part of the region the slate and graywacke are 
associated with younger rocks such as the sandstones, conglomerates, and 
basic lavas of the Keweenawan. In sections 1 and 2, T. 48N., R. 16W., 
a coarse quartzose Keweenawan conglomerate lies unconformably on the 
upturned edges of the slate and graywacke. In the bed of Mission Creek 
and its tributaries, in sections 30 and 31, T. 49N., R. 15W., north of 
Fond du Lac, are several exposures of slate north of the overlying 
Keweenawan sandstone. Outcrops of slate and graywacke also occur 
along Midway Creek in sections 5 and 6, T. 49N., R. 15W., a short dis- 
tance west of the border of the lava flows extending northward from 
Duluth. 

The outcrops throughout the St. Louis River valley are made up of 
alternate layers of slate and graywacke, varying in coarseness of grain 
and ranging from light gray to black in color. Locally dikes of dark 
igneous rocks are abundant. The common rocks are fine-grained, dark 
gray to black slate, some of which is highly siliceous; gray, somewhat 
micaceous slate with well-developed cleavage, and fine-grained to medium 
coarse-grained graywacke, some showing cleavage and some being mas- 
sive bedded and siliceous with little or no schistosity. In some of the 
coarser beds quartz grains are abundant and a few fragments of slate 
are locally present. Concretions or nodules of carbonate minerals occur 
both in the slate and graywacke. They are generally parallel to the bed- 
ding and locally they merge into each other, forming continuous bands. In 
places the longer axes of the concretions are parallel to the cleavage and 
stand at an angle to the bedding. On weathering they leave pod-like 
cavities, stained a rusty brown or black. (Plate X A.) 

Several sets of intersecting joints cut the rock into polygonal blocks 
of various sizes. Faulting has taken place along some of the joints, as 
shown by breccia zones and slickensided surfaces. Thin films of quartz 
and in places calcite, coat some of the joint planes. Quartz occurs as 
veins from a fraction of an inch to several feet in width in many parts of 
the area and also as lenses between slate beds on the crests of smaller folds 
pinching out along the limbs. Pyrite and siderite are often associated 
with the quartz and they also occur as small crystals disseminated in 
the rock. 

Dikes of dark igneous rock, varying from a few inches to 60 feet or 
more wide, cut the slate series in many places. Several are well exposed 
at the Thomson dam. The rock is mainly a fine-grained, greenish black 
diabase. At the contact with the slate or graywacke it is extremely fine- 
grained and dense. Away from the contact it becomes coarser and takes 



90 



GEOLOGY OF EAST CENTRAL MINNESOTA 



on a marked ophitic texture. The dikes strike, in general, a few degrees 
east of north and stand vertically. At the contact the metamorphosed 
sediments have been hardened and their cleavage destroyed, while a new 
cleavage is developed in them parallel to the contact of the dike. This 
condition extends out from the dikes on either side for a few inches to 
several feet. The dikes are more readily broken down by weathering 
than the slate or graywacke, and are generally found in depressions. 

The rocks in the Carlton-Cloquet region lie in a series of low, broad 
folds, with minor folds and crenulations superimposed on the limbs of 
the larger folds. The major structure consists of several large, gently 
eastward pitching anticlines and synclines. The east-west trending rock 
ridges so characteristic of the area around Carlton, are usually formed 
along the limbs of folds, one of the slopes being approximately parallel 
to the dip of the bedding and the other slope cutting across the bedding 
and being steep and ragged. Occasionally ridges are found which are 
structurally crests of anticlines or troughs of synclines. The strike of 
the bedding varies with the position of the bed with reference to the 
fold, but in general it ranges between N.8o°W. and N.8o°E. The dip 
of the beds varies from horizontal to vertical both north and south. The 
strike of the major cleavage varies a few degrees on either side of east and 
west and the dip is at varying angles both north and south, but is nearly 
always steeper than the bedding. The axial lines of the minor folds 
trend about east and pitch at low angles either eastward or westward. 

The northernmost outcrop of slate in east central Minnesota, as de- 
scribed by Spurr. 70 occurs in the vicinity of Brookston in southern St. 
Louis County. The outcrop is said to be located south of St. Louis River, 
in of SW l A of section 27, T. 51N., R. 19W. The rock is de- 

scribed as consisting of alternate layers of siliceous slate, fine-grained 
slate, and sericite schist or slate, the layering being due to the difference 
in coarseness of the original sediments. The schistosity or cleavage is 
about at right angles to the bedding. The cleavage strikes about N.70°E. 
and has a steep dip. The outcrop has not been seen by the writers. 

OUTCROPS OF SANDSTONE AND OTHER CLASTIC ROCKS 
KEWEENAWAN SANDSTONE AND RELATED ROCKS 

Outcrops of pink and red sandstone are abundant in east central 
Minnesota, occurring at intervals along a belt extending southwestward 
from the southwestern end of Lake Superior through Carlton, Pine, and 
Kanabec counties. These rocks have been generally described as be- 
longing to the Lake Superior sandstone, the character and age relations 
of which have already been discussed. (See pages 13 and 14.) 



w Spurr, J. E., The stratigraphic position of the Thomson slate: Am. Jour. Sci., 3d »er., 
vol. 48, p. 164, 1894. 



ROCKS OF EAST CENTRAL MINNESOTA 



9i 



The most southwesterly of the sandstone exposures is described by 
Upham 71 as occurring on Snake River in the NW^ of section 3, T. 
39N., R. 24W., Kanabec County. The sandstone is said to be exposed 
along the southwest side of the river for a distance of 300 feet with a 
width of 75 feet. It rises from 5 to 10 feet above the river, in places 
having a vertical outcrop. It is a coarse-grained sandstone of gray to 
brown color, showing thin layering and containing local beds of dull 
red sandstone. Pebbles varying up to several inches in thickness occur 
scattered through it. Fossils have not been found. The rock has a low dip 
to the east, 15 being the maximum. 

The next sandstone outcrop to the north is one reported as occurring 
in the southern part of T. 40N., R. 23W. 72 It is mapped as extending 
along Snake River and is said to consist of interbedded red sandstone 
and ash-colored clay. 

Aside from these two exposures, no sandstone is known along Snake 
River south of that mentioned on page 43 as occurring in section 23, T. 
42N., R. 23 W., just south of the big bend in the river. The largest ex- 
posure in this locality is found on the west bank of the river and forms 
a small cliff for a distance of about 300 feet along the stream. It con- 
sists of interlayered coarse-grained sandstones of slightly varying texture 
and color. Most of the rock is light-colored pinkish gray to pink, and 
massive; but near the base of the cliff a thin layer of dark red shaly 
sandstone occurs. Locally small pebbles of quartzite and other material 
are found in some of the layers. The general dip of the beds is io° to 
I5°SE. A second outcrop occurs about 500 feet farther downstream on 
the east bank. This exposure is small and consists of sandstone similar 
to that described. 

The sandstone outcropping along Snake River was all mapped by 
Upham as "Potsdam" sandstone. Although no fossils were found in it 
which might aid in determining its position, it was believed to underlie 
the Upper Cambrian and to be of Lower Cambrian age. 

Numerous exposures of sandstone are mentioned by Upham 73 as oc- 
curring along Kettle River southward from Willow River in the western 
part of Pine County, and also along St. Croix River in the southern part 
of Pine County. Most of these outcrops were designated as "Hinckley" 
sandstone, but one or two were mapped as "Potsdam" sandstone. The 
term "Hinckley" sandstone was applied by Winchell, Upham, and others 
of the Geological and Natural History Survey of Minnesota to sandstone 
beds which are not definitely a part of the Upper Cambrian series, but 



71 Upham, Warren, The geology of Mille Lacs and Kanabec counties: Geol. and Nat. Hist. 
Survey of Minn., Geol. of Minn., vol. 2, 1882-1885, pp. 621-622. 
"Upham, W., op. cit. 

78 Upham, Warren, The geology of Pine County, op. cit., pp. 637-642. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



which are stratigraphically above the so-called "Potsdam" sandstone. In 
some reports it is considered as the lowest member of the "St. Croix" 
and in other reports as the uppermost member of the "Potsdam." The 
"St. Croix" sandstone of the early reports embraced the lower portion 
of the Upper Cambrian. 

Exposures of the same sandstone are mapped by Winchell as occur- 
ring along the upper part of Kettle River west of Moose Lake in Carlton 
County, while the abundant exposures of sandstone and conglomerate 
occurring along St. Louis River west of Fond du Lac in southern St. 
Louis County are designated by Winchell as Puckwunge conglomerate 
and "Western" sandstone. 74 All of these beds are believed to be strati- 
graphically below the lowest fossiliferous "St. Croix" beds which are 
found along St. Croix River above and below Taylor's Falls, the Puck- 
wunge conglomerate, which was classed as "Potsdam," being the lowest. 

The sandstone which is found in numerous exposures along Kettle 
River in Pine County is described by Upham as occurring in bluffs along 
the river banks, and also forming the river bed in many places and pro- 
ducing rapids. For a long distance north and south of Sandstone the 
river flows through a great gorge in which sandstone is exposed both 
along the river banks and along the tops of bluffs 75 to 100 feet above 
the river. The rock is mostly fine-grained sandstone, but coarse-grained 
phases occur which, however, are rarely conglomeratic. It generally 
crumbles easily, and as a rule varies in color from buff to gray, though 
red and brown phases occur also. The beds lie flat or dip in various 
directions at low angles. 75 

Exposures of sandstone are found along St. Croix River in the south- 
eastern part of Pine County, above and below the mouth of Snake River, 
as well as a short distance above the mouth of Kettle River. The ex- 
posures near the mouth of Snake River are mapped as "Hinckley" sand- 
stone, and are said to be of friable gray and white sandstone with locally 
beds of conglomerate and of red, green, and yellow shale. The rocks 
are distinctly bedded, being horizontal, or having a slight inclination to 
the south. They are believed to belong to a higher stratigraphical hori- 
zon than the sandstone found along Kettle River. The outcrops north 
of the mouth of Kettle River are designated as "Potsdam," and are said 
to consist of red sandstone with underlying conglomerate. 

The sedimentary rocks found along St. Louis River in Carlton and 
St. Louis counties are divided by Winchell 76 into two stratigraphic hori- 
zons. The lower beds are designated as Puckwunge conglomerate, and 

•* Winchell, N. H., The geology of southern St. Louis County: Geol. and Nat. Hist. Sur- 
vey of Minn., Geol. of Minn., vol. 4, 1896-98, PI. 66. 

Winchell, N. H., The geology of Carlton County, op. cit., pp. 13-16. 
n Upham, W., op. cit. 



ROCKS OF EAST CENTRAL MINNESOTA 



93 



are believed to be of Lower Cambrian age. Exposures occur along St. 
Louis River and north of it at various points. The formation is said to 
be about 100 feet thick, and to grade from coarse conglomerate at the base 
upward into white sandstone and light-colored shale, and then into red 
shale which is overlain by rocks of eruptive origin. 77 The latter are 
designated as the "Manitou" eruptives of late Keweenawan age, and are 
said to stratigraphically underlie the so-called "Western" sandstone. This 
rock consists mainly of red sandstone and conglomerate which grade up- 
ward into lighter-colored sandstone. Its upper portion is believed to 
represent the so-called "Hinckley" sandstone. In the recent work by 
Thwaites already referred to (see pages 13 and 29), the sandstones along 
St. Louis River have all been placed in the Bayfield group of the upper 
Keweenawan. 

Besides the sandstone outcrops mentioned by Winchell and Upham, 
a number of scattered occurrences of sandstone in southern Carlton and 
northern and western Pine counties are mapped by Hall. 78 No descrip- 
tions of the localities, however, are given. 

The relative stratigraphic positions of the various beds of sandstone 
occurring in St. Louis, Carlton, Pine, and Kanabec counties is unknown. 
If the fossiliferous Upper Cambrian series rests conformably upon the 
red clastic series in southern Minnesota, as seems probable, and if the 
latter is supposed to be equivalent to the Keweenawan pink and red sand- 
stone of east central and northern Minnesota, which in earlier reports 
has been called "Hinckley" sandstone, "Western" sandstone, "Lake Su- 
perior" sandstone, etc., then the rocks exposed in the southern part of 
Pine County can probably be regarded as showing the transition from 
the lower part of the Upper Cambrian into the upper part of the Ke- 
weenawan sandstone (red clastic series?). The question as to whether 
the so-called "Hinckley" sandstone belongs to the Upper Cambrian series 
or to the Keweenawan can not yet be answered, nor, as has been men- 
tioned before, do we know whether the sandstone southwest of Lake 
Superior is Cambrian or Keweenawan in age, although the latter view 
seems to be the more probable. 

CRETACEOUS SEDIMENTS 

Besides the outcrops of the older sandstones just described in the 
eastern portion of the region under consideration, there are a number 
of scattered outcrops of slightly consolidated sediments occurring in the 
southwestern portion of the region. These, because of their softness, 

"Winchell, N. H. f The geology of Carlton County, op. cit. 
77 Winchell, N. H., op. cit., p. 13. 

"Hall, C. W., Keewatin area of eastern and central Minnesota: Bull. Geol. Soc. Atner., 
vol. 1 a, PI. 29, 1 90 1. 



94 



GEOLOGY OF EAST CENTRAL MINNESOTA 



do not yield extensive exposures, but locally, as along Mississippi River 
and in the region to the west, scattered outcrops show their presence. 
Mosasaur teeth and other fossils have been found in them in places and 
indicate that they are of Cretaceous age. 

Only one of these outcrops was examined by the writers, this occur- 
ring on the west bank of Mississippi River, just above the mouth of Two 
Rivers in the southern part of Morrison County. Much of the material 
at this locality consists of unconsolidated or slightly consolidated gray 
clay locally yellow or brown iron-stained. Some beds of fairly compact 
shale and siliceous shale occur, however. These are light gray to dark 
gray in color and locally greenish. Very commonly they are impregnated 
with limonite so that they have a yellow to dark brown color. Much of 
the material has curious little concretionary pellets scattered through it. 
In places these are so abundant as to make up most of the rocks, while 
elsewhere they are entirely lacking. Where the rock is iron-stained 
the pellets are dark brown. Elsewhere they are green or gray. 

Other occurrences of Cretaceous sediments described by Upham 79 are 
in the southern part of Stearns County near Torah (Richmond). Three 
miles north of Torah (Richmond) in a ditch dug for drainage, yellow 
and blue clay were encountered with three seams of lignite from i to 6 
inches thick. Locally pyrite was found in the blue clay. Southwest of 
Torah (Richmond) along Sauk River, considerable prospecting is said 
to have been done for lignite. Several shafts were sunk ranging in depth 
up to 125 feet. Most of the material encountered consisted of blue, white, 
and yellow clays in which were local beds of gray to black shale and 
seams of lignite several inches thick. Fossil fragments of mollusks and 
fishes were found in some of the shale beds. In places, crystals of 
selenite were found in the blue clay near the lignite seams. Granite was 
found to underlie the Cretaceous sediments. 

THE CUYUNA IRON-ORE DISTRICT 

LOCATION OF THE DISTRICT 

The Cuyuna iron-ore district is located in the central part of Minne- 
sota, trending southwesterly from the center of Aitkin County through 
Crow Wing and Morrison counties, into Todd County. The most 
important part of the iron-bearing belt is in Crow Wing County. This 
part has a course approximately parallel to Mississippi River and 
lies southeast of it. In the southwestern part of the district, between 
Brainerd and Little Falls, Mississippi River, changing its course from 



n Upham, Warren, The geology of Stearns County: Geol. and Nat. Hist. Survey of Minn., 
Geo!, of Minn., vol. 2, 1882-1885, pp. 459-461. 



THE CUYUNA IRON-ORE DISTRICT 



95 



southwest to south, crosses the ore-bearing belt. Southwest of this point 
the ore-bearing belt continues in the same general direction through 
Morrison County and into Todd County, beyond which it is not known. 
Northeastward, the iron-bearing belt runs from Crow Wing County into 
Aitkin County and disappears beyond Rice River. 

The district has a length northeast and southwest of about 65 miles, 
and ranges in width from a mile or two to as much as 12 miles, not in- 
cluding the Emily area in northern Crow Wing County where iron- 
bearing rocks have been found in scattered localities. The region pre- 
sented for exploration amounts to approximately 40,000 acres. 80 Much 
of this area is under cultivation, farms being scattered throughout. 
Forested and brush lands also occupy large areas, and locally swamps 
occur along the iron-bearing belts. 

Although Mississippi River flows through the district, the drainage is 
very imperfect. Lakes are very abundant throughout the region. The 
largest of these is Mille Lacs Lake, 14 miles wide by 16 miles long, situ- 
ated about 14 miles southeast of Deerwood. The lakes are especially 
numerous in the eastern part of the district, where extensive moraines 
occur, and decrease in number toward the southwest where outwash 
plains predominate over moraines. 

The valley of Mississippi River is narrow and in general is not very 
deep, although locally as at Brainerd there is a distinct gorge. Its only 
large branch in the region under discussion is Crow Wing River, which 
empties into it from the west about 10 miles below Brainerd. Small side 
streams, however, are abundant, but these, as a rule, have worn their 
valleys only short distances back from the Mississippi. 

The topography of the district is predominantly morainic. Over large 
areas it is hummocky, numerous small hills being interspersed with 
swampy areas and lakes. In other places there are extensive outwash 
plains and the surface is level or gently undulating. The greatest differ- 
ence in elevation is about 200 feet. The elevations vary from 1,150 feet 
above sea level, which is approximately the elevation of Mississippi River 
in the western part of the district, to about 1,350 feet, which is the eleva- 
tion of some of the higher hills in the eastern part of the district. The 
highest point in the region adjacent to the Cuyuna district is Warren 
Hill, just west of Mille Lacs Lake, the summit of which is 1,450 feet 
above sea level. Mille Lacs Lake has an elevation of 1,249 ^ eet - 

The principal towns along the ore-bearing belt from northeast to 
southwest are Kimberly and Aitkin in Aitkin County ; Deerwood, Cuyuna, 
Crosby, Ironton, Manganese, Trommald (formerly Iron Mountain), 



"°Zapffe, Carl, The Cuyuna iron-ore district of Minnesota: Supplement to Brainerd 
Tribune, Jury i, iqio. 



g6 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Riverton, Woodrow, Brainerd, and Barrows in Crow Wing County ; and 
Randall in Morrison County. Of these towns, all those located in Crow 
Wing County except Deerwood and Brainerd were built since the begin- 
ning of the development of the iron mines. The oldest of these, Cuyuna, 
is about nine years old. The largest city in the district is Brainerd, which 
has a population of about 8,000 and which at one time was one of the 
lumber centers of northern Minnesota. It was also here that the general 
offices of the Northern Pacific Railway were located before they were 
moved to St. Paul. At the present time Brainerd is the center of a fairly 
prosperous and constantly growing farming community. Of the other 
cities on the range, Aitkin also is well established and is independent of 
the iron-mining industry. Kimberly, Deerwood, and Randall are smaller 
towns, built before iron ore was discovered in the region. All the other 
towns mentioned are more or less dependent on the iron-mining industry 
to which they owe their establishment. 

Two railroads serve the Cuyuna district, the Duluth-Brainerd branch 
of the Northern Pacific Railroad, built in 1870, and the Cuyuna Range 
branch of the Minneapolis, St. Paul, and Sault Ste. Marie Railroad, built 
in 1910. The former runs from Duluth and Superior westward through 
Carlton, Aitkin, Deerwood, and Brainerd, to Staples where it joins the 
main line of the Northern Pacific to the West Coast. In its passage 
through the iron-ore district, it runs approximately parallel to the ore- 
bearing belts. A branch line of the Northern Pacific also runs along 
the east side of Mississippi River between Brainerd and Little Falls con- 
necting at the latter point with the main line to Minneapolis and St. Paul. 
A number of small branches run to various mines in the iron-ore district. 

The Cuyuna Range branch of the Minneapolis, St. Paul, and Sault 
Ste. Marie Railroad leaves the Duluth-Winnipeg branch of this road at 
Lawler and runs westward through Aitkin to Iron Hub. Here it divides, 
a northern branch going to Manganese and a southern branch through 
Cuyuna, Crosby, and Ironton, to Riverton. Various mines are connected 
with the main lines by branch railways. 

HISTORY OF THE DISTRICT 

That iron ore might be found in east central Minnesota was sus- 
pected long before its actual discovery. It is said that when the Northern 
Pacific Railway surveys for the present road were being made previous 
to 1870, through what is now the Cuyuna district, indications of the 
presence of iron ore were found, 81 due probably to compass disturbances 
being noted. Mention of variations in th« direction of the compass needle 



•Kellogg, L. O., Notes on the Cuyuna range, I and II: Eng. and Min. Sou*., to!. 96, pp. 
1 199-1203, Dec. 27, 1913; vol. 97, pp. 7-10, Jan. 3, 1914. 



PLATE XII 




THE CUYUNA IRON-ORE DISTRICT 



97 



in parts of Crow Wing County are said to be found also in the notes 
of the government land surveyors made in the 70's, at which time the 
Marquette Range of northern Michigan was the only iron-ore producing 
district in the Lake Superior region. 

It was about 1890 when the first magnetic surveys were made in east 
central Minnesota for the purpose of locating areas of abnormal mag- 
netic attraction, such as are assumed to indicate the presence of iron 
ore. From this time until the actual discovery of the existence of iron- 
bearing formation in the Cuyuna district, during the summer and fall 
of 1904, magnetic surveys were made at intervals in different parts of 
the region. Some time elapsed, however, before the regularity and ex- 
tensiveness of the magnetic variations were realized, and after this, 
systematic exploration work progressed rapidly. Most of the early mag- 
netic work was done by Mr. Cuyler Adams to whom belongs a large 
share of the credit of discovering and developing the range. It was 
Mr. Adams also who started the first drilling operations on the range ; 
in fact, it was not until he had encountered iron-bearing formation with 
a churn drill that the existence of the new iron range became common 
knowledge. 

Mr. Adams first became interested in the region now known as the 
Cuyuna district early in the 9o's, when the Mesabi district was still a 
wilderness. 82 His attention had been drawn to the area by the fact that 
he noted magnetic variations on certain lands which he had purchased, 
and at various times he made magnetic surveys with dial compass and 
dip needle in attempts to locate lines of maximum magnetic attraction. 83 
In this he was successful, and eventually two such lines were located, 
both having northeast-southwest trends. One of these lines was 12 to 
15 miles long and was located south of Deerwood, in general running 
parallel to and south of the Northern Pacific Railway. It extended from 
the west line of section 10, T. 46N., R. 28W., southwesterly across the 
corners of T. 46N., R. 39W., and T. 45N., R. 29W., to the center of the 
west line of section 29, T. 45N., R. 30W. The other line was found about 
7 miles north of Deerwood, running in a northeast-southwest direction 
across section 30, T. 47N., R. 28W. south of Rabbit Lake. Both lines 
seemed to feather out at the ends. 

Drilling was begun in the district in May, 1903, 84 the first holes being 
put down in the S WJ4 of the SE*4 of section 16, T. 46N., R. 28W., near 
the northeast end of the southern line of attraction, while later holes 
were put down in sections 21, 20, and 10, T. 46N., R. 28W., also on the 



82 Thomas, Kirby, A new promising Lake Superior iron district: Mitt. Wld., vol. 21, pp. 
446-448, Nov. s, 1904. 

83 Adams, Cnyler, Personal correspondence. 

84 Thomas, Kirby, op. ext. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



southern line of attraction. Exploration work was begun in section 30, 
T. 47N., R. 28W., south of Rabbit Lake on the northern line of attrac- 
tion, in July, 1904. Five drills were active throughout the summer of 
1904, and by October, 1904, twenty-two holes, more than half of which 
encountered iron-bearing formation or lean iron ore, had been drilled, 
and the information gained was first given to the public. When the in- 
formation began to spread, much interest and excitement was created, 
even though ore in commercial quantity had not yet been found. 

Besides this drilling in the central part of the district, it is stated that 
drilling was done also during the summer of 1904 in the Dam Lake 
region about three miles south of Kimberly. 85 

Magnetic exploration work progressed very rapidly after the dis- 
covery of iron-bearing formation, and soon the district in which abnormal 
magnetic attractions were found to occur was greatly enlarged by ex- 
tensions to the northeast and southwest. Various mining companies 
from the other iron ranges took options on favorably located properties 
and started exploration work. Among the first of these to come into 
the district was the Oliver Mining Company of the United States Steel 
Corporation, which took options on large tracts of land and started 
drilling operations. After considerable drilling and exploration, how- 
ever, this company withdrew from the district and has not since then 
been identified with any of the Cuyuna range operations. Other com- 
panies that came into the district from other ranges are Pickands, Mather 
and Company, Rogers-Brown Ore Company, Tod-Stambaugh Company, 
M. A. Hanna and Company, Inland Steel Company, Ogelbay, Norton 
and Company, and Jones and Laughlin Steel Company. Some of these 
companies have become permanently identified with the district, while 
others witndrew after preliminary exploration work. 

Not long after the discovery of the existence of iron-bearing forma- 
tion in the Cuyuna district, the possible connection between the iron- 
bearing formation of the western end of the Mesabi district and that 
of the Cuyuna district became a subject of considerable interest, and 
soon drilling was started in the region southwest and south of Grand 
Rapids and north and northwest of Aitkin. This drilling has continued 
to the present time, and ore has been found locally in this region. 

The first attempt at underground work in the Cuyuna district was 
made by the Hobart Iron Company of Pickands, Mather and Company, 
which sunk a 120- foot shaft with several hundred feet of cross cuts in 
the SW}4 of the SE# of section 8, T. 45N., R. 29W. This work was 
done in 1905 and the early part of 1906, but the operation was abandoned 
after some months. 



86 Thomas, Kirby, Notes on the geology of a new iron ore district in Minnesota: Mines 
and Minerals, vol. 25, p. 27, August, 1905. 



THE CUYUNA IRON-ORE DISTRICT 



99 



The first permanent mining shaft in the Cuyuna district was sunk by 
Rogers-Brown Ore Company. During the summer of 1907 this com- 
pany started drilling on lands on which they had secured options in sec- 
tions 29 and 30, T. 47N., R. 28W. 86 south of Rabbit Lake. Some of 
these lands had been previously drilled by other companies, including the 
Oliver Mining Company, the latter having secured an option on the prop- 
erties, but having released them because the results of exploration were 
considered unsatisfactory. The indications of the presence of ore, as 
shown by the Rogers-Brown Ore Company exploration work, appeared 
so favorable, that this company took a lease on the properties and started 
sinking its shaft, which has developed into the present Kennedy mine. 
The Oliver Mining Company had already attempted sinking a shaft on 
the tract, but this had caved in at a depth of 25 feet. Previous to start- 
ing their permanent shaft, the Rogers-Brown Ore Company, therefore, 
sunk an exploration shaft less than 100 feet south of the old Oliver 
Mining Company's shaft in order to test the surface. This was put down 
to a depth of 50 feet, and as no great difficulty was encountered, the per- 
manent shaft was started near the southeast corner of lot 5 of section 
30, T. 47N., R. 28W., about half a mile northeast of Cuyuna. The shaft 
was begun as a 10 by 16 foot lath shaft, but considerable quicksand was 
encountered, and it was changed to a drop shaft, which after great diffi- 
culty and nearly a year of uncertainty was finally ledged at a depth of 
112 feet in October, 1908. 87 

In the meantime exploratory drill work had been carried on through- 
out the region so that the district had been greatly extended both to the 
northeast and southwest. By 1909, 2,000 holes had been drilled, and 
iron-bearing rocks with associated ore had been proved to exist in two 
parallel northeast-southwest belts, one north of the Northern Pacific 
Railway, known as the north range, and the other south of the railway, 
known as the south range. 88 In the south range iron-bearing formation 
had been shown to occur as far northeast as Rice River, T. 48N., R. 26W., 
and southwest as far as Mississippi River, T. 43N., R. 32W., a distance 
of about fifty miles. In the north range, iron-bearing formation had 
been found at various localities in T. 47N., R. 28W., T. 47N., R. 29W., 
and T. 46N., R. 29W. In addition to this, iron-bearing formation had 
been found also west of Mississippi River in T, 133N., R. 32W., south- 
west of Staples. 89 However, exploration work was actively continued, 



98 Woodbridge, D. E., Iron ore in Crow Wing County, Minnesota: Eng. and Min. Jour., 
vol. 84, pp. 775-776, Oct. 26, 1907. 

87 An on., A difficult shaft sinking operation at Deerwood, Minnesota: Iron Tr. Rev., 
vol. 43, pp. 772-774. Nov. 5, 1908. 

"Adams, F. S., The iron formation of the Cuyuna Range: Econ. Geology, vol. 5, pp. 
729-740, 1910; vol. 6, pp. 60-70, 156-180, 191 1. 

» Adams, F. S., op. cit. 



IO0 



GEOLOGY OF EAST CENTRAL MINNESOTA 



and during the fall of 1910 forty drills were at work in various parts 
of the range. 

When the iron-bearing rocks in the district were first discovered, 
Brainerd, Deerwood, and Aitkin were the only towns of importance in 
the region, and Deerwood was the only town located in the area where 
the first important deposits of iron ore were discovered. It was not 
long, however, before new townsites were established and Cuyuna, 
Crosby, and I ronton became active, growing towns. Cuyuna was settled 
in 1908, Crosby was platted in 1909 and settled in 1910, while Ironton 
was established in 1910. Later the towns of Riverton, Manganese, 
Woodrow, Barrows, and, more recently, Trommald were settled. 

It was not long after the completion of the Kennedy shaft to bed 
rock, that the construction work on the Minneapolis, St. Paul, and Sauk 
Ste. Marie Railroad branch line into the Cuyuna range was started and 
by December, 1910, it had reached the town of Cuyuna near the Kennedy 
mine, and the grading had been completed into Crosby and Ironton. 90 
A spur had also been constructed to the Kennedy mine, where the shaft 
had been sunk to the lowest level (262 feet) and where a stock pile of 
45,000 to 50,000 tons of ore was awaiting shipment. The first trainload 
of ore from the Cuyuna range was shipped over this road from the 
Kennedy mine in May, 191 1. 

Meanwhile other shafts were being sunk in the north range, and in 
1912 the Armour No. 1 and Armour No. 2 mines near Ironton, and 
the Thompson mine near Crosby, made their initial shipments, increasing 
the list of producers to four. 

The Armour No. 1 mine was opened by the Iroquois Iron Company, 
which sank a circular concrete shaft south of the ore body during 1910 
and 191 1. During 1912 and 1913, 159,859 tons of ore were produced 
from this mine, but in July, 191 3, all operations ceased. 91 The property 
remained idle during 1914, but in January, 191 5, the lease of the Iroquois 
Company was assigned to the Inland Steel Company, who decided to strip 
the western part of the ore body. Stripping operations were commenced 
in the spring of 191 5 and were continued during the summer, remarkably 
rapid progress being made. A total of about 800,000 cubic yards of over- 
burden was removed, much of which was taken through the Pennington 
open pit with which the Armour No. 1 pit is directly connected. Mining 
operations in the open pit began in August, 191 5, and 79,773 tons of ore 
were shipped before the end of the year. During the season of 1916, the 
Armour No. 1 mine was steadily operated. 



••Anon., The Cuyuna range, Minnesota: Eng. and Min. Jour., vol. 90, p. 1214, Dec. 
17, 1910. 

**Anon., Armour mines on Cuyuna range: Iron Tr. Rev., vol. 57, p. 12*3, Dec. *3, 19 1 S- 



PLATE XIII 




B. REMOVING OVERBURDEN BY HYDRAULIC METHOD AT THE HILLCREST MINE, NEAR 
IRONTON. SHOWS GIANT NOZZLE, CENTRIFUGAL PUMP, AND DISCHARGE PIPE 
PHOTO BY CARL ZAPPFE 



THE CUYUNA IRON-ORE DISTRICT 



101 



The Armour No. 2 mine was leased and drilled by the Iroquois Iron 
Company. 92 Exceptionally good ore is said to have been shown up in 
the drilling, and it was soon discontinued and a concrete shaft was sunk 
during 1910 and 191 1. Mining was started in 1912, and 524,825 tons 
of ore were shipped during the seasons of 1912, 1913, and 1914, by the 
Rogers-Brown Ore Company, which operated the property. In January, 
191 5, the lease was assigned, together with that on the Armour No. 1 
property, to the Inland Steel Company, which shipped 333,856 tons of 
ore from this mine during 191 5. The Armour No. 2 mine has been in 
steady operation since it was first started, and some of the best ore 
shipped from the district has come from this mine. 

The Thompson mine was started as an underground mine by the 
Inland Steel Company in 191 1. A shaft was sunk between two ore 
bodies, one on the north and one on the south, and small quantities of 
ore were shipped during 191 2 and 191 3. It was then decided to strip the 
south ore body, and mine the upper part of it by open pit methods. The 
underground workings were closed in June, 19 13, 93 and stripping opera- 
tions were in progress during the latter part of 191 3 and the early part 
of 1914, the first ore being shipped from the open pit during the summer 
of 1914. Since then the Thompson mine has been operated steadily. In 
the fall of 1916 the steam shovels were removed from the pit and milling 
was inaugurated. At about the same time stripping operations were 
started on the north ore body and continued during the spring of 19 17. 

In 1913 five new mines joined the ranks of producing mines. They 
are the Pennington, Ironton, and Cuyuna-Mille Lacs mines near Ironton, 
the Iron Mountain (now Algoma) mine near Manganese, and the 
Barrows mine near Barrows. These mines, with the exception of the Bar- 
rows mine, are located on the north range, and two of them, the Cuyuna- 
Mille Lacs and Iron Mountain mines, are manganiferous iron-ore mines. 
The Barrows mine is located in the western part of the south range. 

The Pennington mine was the first open-pit mine in the Cuyuna dis- 
trict. Excavation was started in January, 191 3, and before the end of 
the year 1,500,000 cubic yards of overburden had been removed and 
101,136 tons of ore had been mined. 94 The first carload of ore was 
shipped August 2, 191 3, and shipping continued until the close of lake 
navigation, about the middle of November. During 1914 the Pennington 
mine was idle, but shipments recommenced in August, 191 5, after the 



99 Op. cit., pp. 1223-1224. 

88 Swanson, August, Annual report of Inspector of Mines, Crow Wing County, Minn., 
Year ending June 30, 1914, p. 8. 

•* Woodbridge, D. E., Changes and outlook in the lake iron-ore trade: Iron Age, vol. 93, 
pp. 15-18, Jan. 1, 1914. 

Anon., The Dean-Itasca and Pennington mines: Iron. Tr. Rev., vol. 57, pp. 1140-1141, 
Dec. 9, 1915- 



102 



GEOLOGY OF EAST CENTRAL MINNESOTA 



completion of the Arniour No. I open pit, and were continued during 

1916. 

The I ronton and Cuyuna-Mille Lacs mines both made small initial 
shipments in 1913. The Ironton mine is located in the northern part of 
the village of Ironton on the southwest extension of the Armour No. 2 
ore body. It was opened in 1912, 95 and 49,150 tons of ore were shipped 
during 191 3 and 1914. In 191 5 the mine was idle and recommenced oper- 
ations only during the latter part of the season of 1916. 

The Cuyuna-Mille Lacs and Iron Mountain mines were the first 
shippers of manganiferous iron ore in the Cuyuna district. The Cuyuna- 
Mille Lacs mine is situated just south of Menomin Lake about a mile 
northwest of Ironton. The shaft was started in October, 191 1, and 
reached bed rock after penetrating about 50 feet of surface drift. In 
1913, 27,300 tons of ore were shipped, and shipments have continued to 
the present time. Until 1916 the Cuyuna-Mille Lacs mine was the only 
important shipper of manganiferous iron ore in the district, only small 
shipments being made from the Iron Mountain mine and from one or 
two of the iron-ore mines. Ore, ranging from about 10 per cent to 30 
per cent in manganese, 96 and between 35 and 40 per cent in iron has 
been shipped from the Cuyuna-Mille Lacs mine, several grades with 
different manganese and iron content being produced. At present the 
lower grade ores are being produced in larger quantities. 

The Iron Mountain mine, located about half a mile south of Man- 
ganese, was opened by the Iron Mountain Mining Company. The first 
shaft was sunk during 191 1 and 1912, but was closed down and a second 
shaft was started in May, 1913. 97 The first ore was shipped in 1913, con- 
sisting of a few carloads used for experimental purposes. 98 The mine 
was idle in 19 14, but operations were recommenced in 191 5, 8,600 tons 
of ore being shipped during the season. In the winter of 1916 the Iron 
Mountain mine was taken over by the Hoch Mining Company and re- 
named the Hoch mine. A large tonnage was mined and shipped during 
the season of 1916. At the present time (summer 1917) the mine is 
being operated by the Algoma Mining Company and is known as the 
Algoma mine. 

The Barrows mine, located 4 miles southwest of Brainerd, was the 
first of the south range mines to join the list of producers. It was 



* Swanson, August, Annual report of Inspector of Mines, Crow Wing County, Minnesota. 
Year ending June 30, 1914, p. 11. 

••Zapffe, Carl, Matters of interest to operators regarding the Cuyuna district: Proc. 
Lake Sup. Mm. Inst., vol. 20, pp. 196 et seq., Sept. meeting, 1915. 

"Swanson, August, Annual report of Inspector of Mines, Crow Wing County, Minnesota. 
Year ending June 30, 1913, p. II. 

•"Zapffe, Carl, op. cit., p. 197. 




A. SINKING A CONCRETE SHAFT AT THE CROFT MINE, NEAR CROSBY 
PHOTO BY CARL ZAPPFE 




B. IRON-ORE CONCENTRATING PLANT AT THE ROWE MINE, NEAR RIVERTON 
PHOTO BY CARL ZAPPFE 



THE CUYUNA IRON-ORE DISTRICT 



103 



soon followed, however, by the Adams mine and more recently by the 
Wilcox and Brainerd-Cuyuna mines. 

At the Barrows mine considerable difficulty was encountered in shaft 
sinking on account of bad ground. A test shaft was sunk during 191 1 to 
a depth of 80 feet, and the conditions encountered were so serious that 
the second shaft was started as a timber drop shaft. This was aban- 
doned after reaching a depth of more than 50 feet. A third shaft, also 
a timber drop shaft, was started early in 191 2 and was finally ledged. 
The Barrows mine was operated by the Virginia Ore Mining Company, 
of M. A. Hanna and Company, the first ore being shipped from the mine 
in 191 3. Shipments continued until the summer of 1914 when the lease 
was abandoned and the mine ceased operating. The Barrows mine is 
owned by the Brainerd Mining Company. 

During the shipping season of 1914 only two new mines were added 
to the producing list, viz., the Rowe mine, near Riverton, on the north 
range and the Adams mine, 3 miles southwest of Deerwood, on the south 
range. Several new shafts, however, were started during the year, in- 
cluding the Sultana shaft about a mile northwest of Ironton, the Croft 
shaft near Crosby, the Wilcox shaft near Woodrow, and the Rowley shaft 
half a mile south of Barrows. 

The Rowe open-pit mine of the Pittsburgh Steel Ore Company was 
excavated during 191 3 and 1914, the first ore being shipped in the autumn 
of 1914. Sluicing and hydraulic stripping were successfully applied in 
the removal of the overburden which covered the ore, this being the first 
instance of such methods being employed in the removal of overburden 
in the Lake Superior district." The operations were begun in August, 
1913, and for a short time the sluicing method was used. The location is 
near the shore of Little Rabbit Lake, and a hill rising to an elevation of 
50 feet above the level of the lake covered the ore body. Water was 
therefore pumped from the lake up to the hill and turned loose, the re- 
turning stream carrying a full burden of material down to the lake. This 
was continued until the slope became too low for the effective removal 
of material. A hydraulic giant and a centrifugal sand pump were then 
installed. The stream from the hydraulic giant undercut the banks of 
sand and clay, and the material was washed by the water down to a sump 
where it was picked up through suction by the sand pump and discharged 
through a 12-inch pipe into the lake. The stripping operations continued 



99 Kellogg, L. O., Stripping with the hydraulic giant: Eng. and Min. Jour., vol. 97, pp. 
166-167, Jan. 17, 1914. 

McAuliffe, P. J., Stripping a mine by hydraulic methods: Coal Age, vol. 5, pp. 568-569, 
Apr. 4, 1914. 

McCarty, E. P., Hydraulic stripping at Rowe and Hillcrest mines on the Cuyuna Range, 
Minn.: Lake Sup. Min. Inst., Sept. meeting, 1915. See also Iron Tr. Rev., vol. 58, pp. 136-139, 
Jan. 13, 1916. 



104 



GEOLOGY OF EAST CENTRAL MINNESOTA 



into the summer of 1914, a total of about 1,581,000 cubic yards of earth 
being removed, of which 81,000 cubic yards were removed by the pre- 
liminary sluicing. The final clearing up of boulders and other material 
not removed by hydraulicking was done with steam shovels. 

During 1914, 80,000 tons of ore were shipped from the Rowe mine. 
But it was found that in many parts of the ore body the ''direct-shipping" 
ore was irregularly mixed with low-grade material which could not be 
shipped profitably without concentration. Concentration tests were made 
on this material during the following winter and during the spring and 
summer of 191 5, a concentrating plant was erected. No ore was mined 
during this time, but in August, 191 5, upon the completion of the con- 
centrating plant, operations recommenced and 143,163 tons were shipped 
before the season closed. A large part of this tonnage consisted of con- 
centrates. 

The Adams mine is the easternmost mine on the south range. The 
area in which it is located was one of the first to be explored in the 
Cuyuna district. In 1908-9, a shaft was sunk on the property by Rogers 
Brown Ore Company, but was abandoned at a depth of 52 feet before it 
had penetrated the surface drift. 100 A few years later the property was 
leased by Cuyler Adams and a concrete shaft was sunk for him by the 
New York Foundation Company in 191 2. The ore body was opened 
during 1913, and some ore was shipped in 1914, but during the fall of 
the latter year operations ceased and the mine has since then been in- 
active. 

The year 191 5 was one of considerable activity in the Cuyuna dis- 
trict. Early in the year the concentrating plant at the Rowe mine was 
started, stripping operations were begun at the Hillcrest mine, about a 
mile west of Ironton, and at the Armour No. 1 mine north of Ironton, 
while shaft-sinking was in progress at the Croft mine near Crosby, and 
at the Rowley mine of the Barrows Mining Company, south of Barrows. 
A little later stripping operations were begun on the Mahnomen open- 
pit mine of the Mahnomen Mining Company, north of the Pennington 
open pit. Toward the latter part of the summer, the Hopkins shaft was 
sunk by Breitung and Company, Ltd., a short distance east of the Cuytina- 
Mille Lacs mine, while in the fall the Tabbert shaft of the Adbar Devel- 
opment Company was started in the south range between Brainerd and 
Woodrow. 

Only three new mines made their initial shipments in 191 5, these 
being the Sultana mine north of Ironton on the north range, the Wilcox 
mine near Woodrow, and the Brainerd-Cuyuna mine near Brainerd, both 
on the south range. The Sultana mine of the Sultana Mines Company, 



100 Zapffe, Carl, Personal communication. 



PLATE XV 




A. PENNINGTON— ARMOUR NO. I OPEN-PIT MINE NEAR IRONTON. LOOKING 
NORTHEASTWARD 
PHOTO BY CARL ZAPPFE 




B. HEAD-FRAME AND TRESTLE OF CUYUNA— MILLE LACS MINE, NEAR IRONTON 

LOOKING EASTWARD 
PHOTO BY CARL ZAPPFE 



THE CUYUNA IRON-ORE DISTRICT 



105 



formerly the Cuyuna-Sultana Company, is located in the southern part 
of section 3, T. 46N., R. 29W., about half a mile east of the Cuyuna- 
Mille Lacs mine, and like the latter is a manganiferous iron-ore mine. 
The first exploration shaft, which is now Shaft No. 1, was begun June 
25, 1914, and the permanent hoisting shaft was begun October 1, 1914. 
Both were ledged during the fall of 1914. A small quantity of ore was 
shipped in the fall of 191 5, and during the spring and summer of 1916 
a considerable tonnage was mined. A small experimental concentrating 
plant for treating the lower grade ore was erected in the spring of 191 5, 
but tests were unsuccessful. 

The sinking of the Wilcox shaft was begun early in 1914 and con- 
tinued during the summer and fall. 101 The ore body was opened during 
the winter and the following spring, shipments beginning during the 
summer. The Wilcox mine is the only one of the south range mines 
which has produced steadily since it was opened. It was operated by the 
Canadian-Cuyuna Iron Mining Company during 191 5 and 191 6. Ini9i7 
it was taken over by the Paterson Construction Company which is operat- 
ing it at present. 

The Brainerd-Cuyuna mine had difficulty in shaft-sinking on account 
of quicksand. Two shafts were sunk, but were abandoned before reach- 
ing bed rock, the first one being started in August, 19 13, and the second 
one in February, 1914. 102 A third one, however, started late in 1914, 
reached bed rock with difficulty in the spring of 191 5. The first ore was 
raised during the summer, the shipments amounting to 3,000 tons. 

During 1916 there was much activity among the manganiferous iron- 
ore mines, and considerable tonnages of this ore were shipped from the 
Cuyuna-Mille Lacs, Sultana, Hoch (now Algoma), and Ferro mines. 
Three new manganiferous iron-ore mines, the Ferro, located near Iron 
Mountain, the Mangan No. 1 mine situated just northeast of the Cuyuna- 
Mille Lacs mine, and the Mangan No. 2 mine about one fourth of a mile 
north of the Armour No. 1 mine near Ironton, joined the list of pro- 
ducers. During the late fall, several new shafts were started on man- 
ganiferous iron-ore bodies in different parts of the north range, among 
them being the shaft of the Merritt interests near Iron Mountain, the 
McKenzie shaft of the Donohue interests near Manganese, and the Joan 
shaft of the Joan Mining Company, east of Menomin Lake. 

The Ferro mine, formerly known as the Duluth-Brainerd, is located 
about half a mile southwest of the Algoma mine already mentioned. A 
shaft was sunk on this property by the Duluth-Brainerd Iron Company 
during the fall of 191 3, and a little ore was taken out. 103 There were 



101 Swanson, August, Annual report of Inspector of Mines, Crow Wing County, Minn. 
Year ending June 30, 19 14, p. 13. 
103 Ibid., p. 12. 
loa Ibid., p. 13. 



106 GEOLOGY OF EAST CENTRAL MINNESOTA 

no important operations, however, during 1914 and 191 5. Early in 1916 
the property was taken over by the Onahman Iron Company, and a large 
tonnage of high-grade manganiferous iron-ore was shipped during the 
season. 

The Mangan No. 1 mine of the Mangan Iron and Steel Company 
was begun in December, 191 5, and a considerable quantity of ore was 
taken out during the summer and fall of 1916. 

The Mangan No. 2 mine, also of the Mangan Iron and Steel Com- 
pany, contains both iron ore and manganiferous iron ore. Shaft sinking 
was begun in June, 19 16, and a small quantity of manganiferous iron ore 
was mined during the autumn. 

Not only were the manganiferous iron-ore mines active during 1916, 
but more iron ore was produced in the district than during any previous 
year. Four new iron-ore mines joined the producing list during 1916, 
these being the Mahnomen and Hillcrest open-pit mines near Ironton and 
the Croft and Meacham underground mines near Crosby. A second 
shaft was started by the Adbar Development Company on the Tabbert 
property near Woodrovv, the first having been abandoned on account of 
bad ground. This shaft, as well as that at the Rowley mine, near Bar- 
rows, had not reached bed rock in the autumn of 1916. Late in the 
autumn of 1916, the Cuyuna Realty Company started sinking a shaft on 
the Feigh property, located between the Pennington and Hillcrest open 
pits. 

The Mahnomen ore body was stripped of its overburden by means of 
steam shovels, operations being started early in the summer of 191 5. The 
work continued during the summer and fall of 191 5, and the spring of 
19 1 6. As the ore body was underneath a swampy area, considerable 
difficulty was encountered in excavating on account of water. However, 
comparatively rapid progress was made, and ore was shipped during the 
summer and fall of 1916. 

The stripping operations at the Hillcrest mine were commenced in 
April, 191 5. 10 * with hydraulic giant and sand pump, such as had been 
used in removing the overburden from the ore body at the Rowe mine. 
The average thickness of the overburden overlying the ore body was about 
sixty-five feet, and it was calculated that approximately a million cubic 
yards of material had to be removed. The work progressed rapidly dur- 
ing the summer and fall, with an operating force of only six men to a 
shift, and it continued through the extremely cold weather of January, 
191 6. During midwinter, however, operations ceased, but were recom- 



104 Anon., Stripping the Hillcrest iron mine with a sand pump: Eng. and Min. /our., 



PLATE XVI 




A. HEAD-FRAME, CONCENTRATING PLANT, AND PART OF OPEN PIT AT THOMPSON 
MINE, NEAR CROSBY. LOOKING NORTHEASTWARD 
PHOTO BY CARL ZAPPFE 




B. HEAD-FRAME AND POWER HOUSE OF ADAMS MINE, NEAR DEERWOOD 
PHOTO BY CARL ZAPPFE 



THE CUYUNA IRON-ORE DISTRICT 



107 



menced in the spring. In the summer the boulders and coarse debris 
left by the sand pump were removed by means of a steam shovel, and 
the first ore was shipped from the mine in October. The Hillcrest mine 
is operated by the Hill Mines Company. 

The Croft mine of the Merrimac Mining Company (John A. Savage 
and Company), is located about half a mile north of Crosby. Shaft sink- 
ing was begun at this mine during the fall of 1914 and continued through 
the summer of 191 5. In the fall of 191 5, cross cuts were driven from the 
shaft northward into the ore body, and during the winter and spring the 
ore body was opened up in preparation for the shipping season. A con- 
siderable amount of ore was shipped during the summer and fall of 1916. 

The Meacham mine is situated a short distance southwest of the 
Croft mine near the edge of the Thompson open pit. The shaft was 
sunk during 1910 and 191 1 at approximately the same time that the shafts 
for the Armour No. 1, Armour No. 2, and Thompson mines were being 
sunk, but the mine was idle until the spring of 1916, the first shipment 
being made by Rogers-Brown Ore Company in May. 

The iron ore and manganiferous iron ore production from the Cuyuna 
district to the end of 1916 aggregated 5,116,358 tons, of which 4,747,268 
tons were iron ore and 369,090 tons were manganiferous iron ore. This 
ore was produced by 21 mines, the bulk of it coming from the Kennedy, 
Armour No. 2, Thompson, Pennington, Rowe, and Armour No. 1. Three 
mines which contributed to this production, viz., the Adams, Barrows, 
and Brainerd-Cuyuna mines, were idle in 1916. The remaining 18, how- 
ever, were actively operated, seven of them making their first production 
in 1916. The seven new mines are the Croft, Ferro, Hillcrest, Mahno- 
men, Mangan No. 1, Mangan No. 2, and Meacham. The accompanying 
table shows the production of the mines of the Cuyuna district to the end 
of 1916, as reported to the Division of Mineral Resources of the United 
States Geological Survey. 

GENERAL GEOLOGY OF THE DISTRICT 
GENERAL STATEMENT 

The only known outcrops of bed rock in the Cuyuna iron-ore district 
are the quartzite and greenstone occurring near Dam Lake and Long 
Lake, a short distance south of the northeastern end of the iron-bearing 
belt, and the chlorite schist at Randall near the southwestern end of the 
iron-bearing belt. Both of these have already been described. Over 
the rest of the region there is a covering of glacial till varying in thick- 
ness from o to 400 feet. The variation in thickness of the mantle of 
glacial material is due in part to the present surface topography and in 
part to the irregularity of the bed-rock surface. The least thickness 



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THE CUYUNA IRON-ORE DISTRICT 



109 



known in the principal part of the iron-ore district is 15 feet. Because 
of this drift mantle the study of the occurrence, structure, and distribu- 
tion of the various rocks in the Cuyuna district is of necessity based on 
information gained by drilling and mining operations. 

The rocks in the Cuyuna district which have been found up to the 
present time can all be grouped under three classes, (1) metamorphosed 
sedimentary and igneous rocks interlayered with each other in beds and 
lenses, usually with steep dip due to extensive folding, (2) igneous rocks 
intruded into the metamorphosed rocks subsequent to their metamorphism 
and deformation, and (3) younger rocks which lie horizontally on the 
eroded surfaces of the older rocks. 

The age of the various rocks is not definitely known. Because of their 
metamorphosed and folded state, the first group mentioned above has 
usually been classed as Huronian, and on account of their general rela- 
tion to the rocks of the Mesabi district and the occurrence of iron-bearing 
formation in them, it has generally been supposed that they are of the 
same age as the iron-bearing rocks of the Mesabi district, i.e., upper 
Huronian. 106 

In accordance with this view all the slates, schists, and other 
metamorphosed rocks in east central Minnesota have been grouped under 
the name of Virginia slate, which term was originally applied to the slate 
overlying the iron-bearing Biwabik formation in the Mesabi district. 
In view of the facts that no rock outcrops exist in the region between 
the southwestern end of the Mesabi district and the northeastern end 
of the Cuyuna district, a distance of nearly 40 miles, and that only scat- 
tered drilling has been done in this region, it seems premature to cor- 
relate definitely the rocks of the two districts. It is possible that the 
metamorphosed rocks of the Cuyuna district may belong to either a higher 
or a lower horizon than the upper Huronian rocks of the Mesabi dis- 
trict, or possibly rocks of several different geologic ages are present. It 
is undoubtedly true that in most areas of pre-Cambrian rocks in northern 
United States and Canada, where detailed work has been possible, the 
occurrence and distribution of the rocks has been shown to be much more 
complex than is at present supposed to be the case in east central Minne- 
sota. Our present knowledge of the details of the structure and stratig- 
raphy of the rocks of the region is still too meager to allow the sub- 
stitution of a definite classification for the one now in use. In fact there 
is no evidence to show that this classification may not be correct. How- 
ever, it would doubtless have been better had a local term been used for 



""Van Hise, C. R., and Leith, C. K., The geology of the Lake Superior region: U. S. 
Geol. Survey Mon. 52, pp. 31 a et seq., 191 1. 



no 



GEOLOGY OF EAST CENTRAL MINNESOTA 



the metamorphosed rocks of the Cuyuna district pending the accumula- 
tion of more evidence. In the text of this report general terms preferably 
are used. 

The old metamorphosed rocks are principally slates and schists of 
various kinds associated with beds and lenses of iron-bearing formation. 
Locally quartzite also has been found, and at one or two places lime- 
stone is reported to occur. The slates include gray argillaceous slate, 
finely micaceous gray phyllite, black carbonaceous slate and dark green 
amphibolitic slate. The schists are principally quartzose and argillaceous 
sericitic schists, which are probably of sedimentary origin. With these 
is associated green chloritic schist, much of which appears to be of 
igneous origin, although the original texture has almost entirely disap- 
peared. Metamorphosed chloritic igneous rocks, in which the schistosity 
is but slightly developed, however, and in which igneous textures are 
still clearly visible, have also been found in drilling operations in different 
parts of the district. 

The rocks of second class, viz., igneous rocks which are intrusive 
into the metamorphosed rocks and have not themselves been metamor- 
phosed, are generally supposed to be of Keweenawan age. If the meta- 
morphosed rocks are of upper Huronian age this is a reasonable assump- 
tion. These rocks occur as dikes or irregular intrusive masses of different 
sizes, cutting the older rocks, and in many instances causing further 
metamorphism along the contacts. All rocks of this class found in the 
district up to the present are subsilicic, and are usually medium fine- 
grained or porphyritic. 

The rocks of the third class occur only locally and in minor quantity. 
They are of two types, (i) horizontal flows of igneous rocks, probably 
of Keweenawan age, which have been found at one or two places under- 
lying the glacial drift and overlying the metamorphic complex, and (2) 
isolated patches of ferruginous conglomerate and other sediments sup- 
posed to be of Cretaceous age, and probably to be correlated with similar 
patches of sediments occurring in the western portion of the Mesabi 
district. 106 The rocks of the first type may perhaps be correlated with 
the Keweenawan trap rocks found in the region of St. Croix River in 
eastern Minnesota. No outcrops of them have, however, been found in 
the central part of the state. The second type of rocks is probably a 
phase of the clayey and calcareous rocks of Cretaceous age, beds of which 
occur outcropping along Mississippi River south of Little Falls and else- 
where in central Minnesota. 



l0 * Zapffe, Carl, The Cuyuna iron ore district of Minnesota: Amer. Ass'n. Adv. Sci., Dec. 
meeting, 1910. See also Supplement to the Brainerd Tribune, July l, 1910. 



THE CUYUNA IRON-ORE DISTRICT 



in 



STRUCTURE OF THE ROCKS 

The rocks of the Lake Superior region suffered folding at several 
periods in their history. As far as is known, however, the forces causing 
the deformation at the different periods have been applied in approxi- 
mately the same directions, i.e., northwest-southeast, so that the results, 
as apparent in the rock structure, are superimposed upon each other. 
The final outcome of these various deformational activities has been that 
all the pre-Cambrian rocks of the Lake Superior region have been folded 
into a great complex synclinorium trending approximately northeast- 
southwest. The Keweenawan rocks have suffered the least folding of 
all, and form a broad trough with a gently dipping northwest limb and a 
more steeply dipping southeast limb. The older rocks have suffered 
successively greater deformation, the upper Huronian having many minor 
folds within the main synclinorium, while the lower Huronian and 
Archean are in places so intensely deformed that it is almost impossible 
to decipher their relation to the general structure of the region. 

Because of the dominance of the northwest-southeast deformation, 
the general strike of the rock structures, such as bedding, cleavage or 
schistosity, and folds, throughout the Lake Superior region, varies be- 
tween east-west and northeast-southwest. It is this general similarity of 
secondary structures in all the rocks which most strongly points to the 
conclusion that the deformational movements at different periods have 
varied but little in direction. 

Conforming with the general structure of the Lake Superior region, 
the rocks of the Cuyuna district are folded into a complex series of anti- 
clines and synclines trending northeast-southwest. 107 In the Cuyuna dis- 
trict the dip of the limbs of the folds is usually vertical or very steep, and 
may be either to the southeast or northwest, the former dip predomi- 
nating. Very often close folding has resulted in producing an approxi- 
mate parallelism of both limbs of a fold. This is probably the explanation 
for the great predominance of southeast dips in the district, which are 
characteristic both in the north and south ranges. The pitch of the folds 
is usually very low and may be either to the northeast or to the southwest. 
Pitches in one or the other direction usually predominate in different 
parts of the district. Thus at the southwestern end of the north range 
the pitch of the folds is in general to the northeast, while along the cen- 
tral part of the north range it is to the southwest. Frequently the pitch 
is so low that the crests of anticlines are practically horizontal for miles. 
Because of this low pitch of the folds, the various rock layers appear on 



*»Van Hiae, C. R., and Leith, C. K., The geology of the Lake Superior region: U. S. 
Geol. Survey lion. 52, pp. 620 et seq., 191 1. 

Zapffe, Carl, The Cuyuna iron ore district of Minnesota: Amer. Ass'n. Adv. Sci., Dec 
meeting, 191a. See also Supplement to the Brainerd Tribune, July i, 1910. 



112 



GEOLOGY OF EAST CENTRAL MINNESOTA 



the erosion surface as approximately parallel northeast-southwest trend- 
ing bands. Locally, where the pitch brings rock layers below the erosion 
surface, sharp turns occur and the bands double back on themselves. 
Elsewhere minor drag folds cause local irregularities in their trend. 

On account of the lack of exposures and as yet insufficient drilling 
and underground development, it has not been possible to work out the 
details of the structure. Drag folds and other secondary structures point 
to the existence of several major and many minor folds in both the north 
range and the south range. Ultimately, by means of these structures, it 
may be possible to determine the relationship between the rocks in dif- 
ferent parts of the district. The apparent absence of any definite strati- 
graphic succession, however, is a serious drawback in working out the 
geologic relations. 

The general distribution and structure of rocks in east central Minne- 
sota indicates that the Cuyuna district is situated near the axis of the 
southwestern extension of the Lake Superior synclinorium. The close 
folding, such as exists in this district, would thus be naturally accounted 
for as well as the very gentle folding along the Mesabi range which is 
supposed to be on the north limb of the synclinorium. The south limb, 
represented by the Penokee-Gogebic district in northern Michigan and 
northern Wisconsin, is as yet unknown in Minnesota. The extensive 
areas of granitic rock in the Saint Cloud and Mille Lacs Lake regions 
may be south of the south limb of the synclinorium, or, as believed by 
Zapffe, 108 they may represent intrusions within the synclinorium. It is 
known that some of these granites do intrude metamorphosed sediments, 
but the age of the latter is not definitely established. If the granite is 
lower-middle Huronian or Laurentian in age and represents the basement 
upon which the metamorphosed sediments of the Cuyuna district rest, 
then the south limb of the synclinorium may be expected to run northeast- 
ward from Little Falls through the northern part of Mille Lacs Lake 
toward Kettle River. On the other hand, if the granite is of later age 
than the metamorphosed sediments and represents intrusions within it, 
the south limb of the synclinorium may be far to the southeast. 

LITHOLOGY OF THE ROCKS 
OLDER METAMORPHOSED ROCKS (VIRGINIA SLATE) 

Geologic relations. — The principal constituents of the metamorphosed 
rocks of the Cuyuna district are chloritic, micaceous, and quartzose 
schists. Associated with these are extensive layers of iron-bearing rock 
and locally lenses of quartzite and masses of metamorphosed igneous 
rock. Black, carbonaceous slate and dark green amphibolitic slate also 



108 Zapffe, Carl, op. cit. 



THE CUYUNA IRON-ORE DISTRICT 



113 



occur in many places, and limestone has been encountered locally . These 
rocks are irregularly interlayered with each other. They all have a general 
northeast-southwest strike, and usually a steep dip to the southeast or 
northwest, the former dip being by far the more common. On the 
horizontal surface the different rock layers appear as discontinuous bands 
of varying width all approximately parallel. 

Up to the present time it has been impossible to work out any definite 
stratigraphic succession for the metamorphosed rocks of the district. The 
only horizontal markers of any considerable extent are the layers of iron- 
bearing rock. These, however, are unsatisfactory since several distinct 
bands of iron-bearing rock run through different parts of the district, and 
it is not known as yet whether they are the surface expressions of a single 
layer which has been complexly folded, or whether several different layers 
and lenses of iron-bearing rock exist at different stratigraphic horizons. 
The different bands of iron-bearing rock differ in the character of ma- 
terials which they contain and in the nature of the wall rock which 
bounds them. This would indicate that they are not portions of the same 
layer. On the other hand, the same band of iron-bearing rock may show 
distinct and even abrupt changes in the nature of the materials composing 
it, both along the strike and across it. Thus certain bands of iron-bearing 
rock show highly manganiferous ore along one side and ferruginous 
chert or hematite with only a trace of manganese along the other side. 
Other bands of iron-bearing rock consist almost entirely of red or brown 
hematite and hematitic or limonitic chert in one locality, while at another 
locality the same band may consist largely of black magnetic ore and 
magnetitic slate. It is thus difficult to correlate the different bands on 
the basis of lithology. 

Some geologists believe that probably the different bands of iron- 
bearing rock of the north range may be referred to a single layer, but 
that the bands in the south range represent a different and probably 
higher stratigraphic horizon. This assumption is based on the fact that 
the ore and iron-bearing rock in the two ranges present rather distinct 
lithologic differences. The south range ore is usually black or yellow 
and is associated largely with slaty, argillaceous rocks, many of which 
are magnetitic. Ferruginous chert does not occur as abundantly as on 
the north range and manganese oxide is present only in small amounts 
in a few places. The north range ores on the other hand are usually 
brown, red, or blue, and are abundantly associated with ferruginous chert 
and only locally with amphibole-magnetite rock or magnetitic slate. They 
also contain much manganese oxide in places which may be present in 
distinct bodies or may be disseminated through the iron ore. In view 
of the facts, however, that much of the north range iron-bearing rock 
has associated with it slaty, magnetitic rocks, and that the manganese 



GEOLOGY OF EAST CENTRAL MINNESOTA 



oxide is only local in its occurrence and apparently increases in amount 
toward the north, it does not seem that the lithologic differences noted 
between the north range and south range iron-bearing rocks are sufficient 
to justify their assignment to separate horizons. On account of the 
unique character of the iron-bearing beds, it is believed that somewhat 
peculiar conditions attended their deposition. That such conditions were 
frequently repeated in the course of sedimentation does not seem prob- 
able in the absence of definite data to the contrary. It is, therefore, more 
reasonable for the present to assume that only one main iron-bearing 
horizon exists in the Cuyuna district. Cheney 109 has recently made an 
interpretation of the rock structure of the district on this supposition. 

What is true of the layers of iron-bearing rock as horizon markers 
is also true of the associated rocks, and as these in general are much 
less continuous and less uniform, they are of but little value in this re- 
spect. The chloritic schist, which is perhaps the most nearly uniform of 
these associated rocks, occurs in lenses, which, although of considerable 
width, are rarely of great longitudinal extent. Such lenses are found at 
various horizons, frequently being interlayered within the iron-bearing 
rock itself. 

Carbonaceous slate is frequently used as a local horizon marker as it 
has often been found to occur along the boundary of iron-bearing beds. 
However, single layers are rarely continuous for long distances along the 
strike. On the south range, black carbonaceous slate is much more char- 
acteristic than on the north range, being so commonly associated with 
the ore bodies that its presence is considered a good indication for ore. 

It might be supposed that quartzite would form a definite stratigraphic 
unit in this district as it does in most of the other Lake Superior iron-ore 
districts. However, in the Cuyuna district, quartzite lenses and layers 
have been encountered in only a few places, and it has not yet been pos- 
sible to determine any relationship between the scattered occurrences. 

It is possible that, as exploration work progresses, and individual beds 
of different rocks are traced along the strike, some orderly arrangement 
of layers may be found to exist. For the present, however, any attempted 
correlation is largely speculative. 

Deerzvood iron-bearing member. — The iron-bearing rocks of the Cuy- 
una district present a variety of lithologic types. Among the more com- 
mon types are hematitic and limonitic chert, hematitic and limonitic slate, 
cherty and argillaceous ferrous carbonate, siliceous magnetitic slate, 
amphibole-magnetite rock, green amphibolitic slate, jaspilite, dark blue, 



10 * Cheney, C. A., Jr., Structure of the Cuyuna iron-ore district of Maoocoote : Eng. and 
Min. Jour., vol. 99, pp. 1113-1115, June 26, 1915. 



PLATE XVII 




THE CUYUNA IRON-ORE DISTRICT 



US 



red, brown, black, and yellow iron ore, black, red, and brown manganifer- 
ous iron ore, green chloritic schist, and dark red hematitic schist. 

Limonitic and hematitic chert are the most characteristic iron-bearing 
rocks in the Lake Superior region. In the Cuyuna district they are most 
abundant in the north range where they compose the principal part of the 
iron-bearing layers. However, they are common on the south range as 
well. They usually consist of interlaminated white, pink, or gray chert 
and hematite or limonite, the chert and iron oxide occurring in alternate 
layers. (Plates XVII and XVIII.) In some parts of the rock the hematite 
or limonite laminae are fairly pure, and elsewhere they are very siliceous. 
In many places the iron oxide present is merely a stain or an impregna- 
tion. Thus there are all gradations of ferruginous chert from siliceous 
iron ore to chert containing only a small percentage of iron oxide. Much 
of the ferruginous chert, instead of being banded, presents a blotchy 
appearance due to the irregular distribution of chert and iron oxide or 
to the irregular staining of the chert by the iron oxide. The more sili- 
ceous iron-stained chert usually shows a more irregular distribution of 
chert and iron oxide. In many places, however, very siliceous ferruginous 
chert shows an even brown or red color throughout, the entire mass of 
chert being impregnated with iron oxide. 

The chert laminae in ferruginous chert are typically fine-grained, 
dense, and flinty, but locally they have suffered partial disintegration and 
present a .fine sugary appearance. When completely disintegrated they 
break up into fine sand, giving rise to what is known as wash ore. Wash 
ore is soft ferruginous chert from which the disintegrated chert can be 
removed by washing while the iron oxide remains behind and becomes 
concentrated. 

A microscopic examination of ferruginous chert shows it to consist 
of little else than silica and iron oxide. A large proportion of the silica 
present is in the form of slightly recrystallized chert, while the rest is 
sufficiently coarsely crystalline to be called quartz. Very little chert is 
amorphous as are cherts of recent geologic age. Most of it has a micro- 
crystalline texture, the coarseness of which varies so that all gradations 
occur between chert and quartz. The iron oxide is usually amorphous, 
generally occurring thickly bunched along layers, but at many places also 
being disseminated through the chert as tiny specks. Locally phases of 
the ferruginous chert contain iron oxide in the crystalline form. 

Hematitic and limonitic slates are not as abundant in the district as 
ferruginous chert, but occur locally as beds and lenses interlayered with 
ferruginous chert or iron ore. They are usually dark red or light to 
dark brown and present a thinly laminated appearance. Red hematitic 
slate is by far the most abundant. It is dark red, soft, and in general is 
finely and regularly laminated. The brown limonitic slate is usually more 



n6 GEOLOGY OF EAST CENTRAL MINNESOTA 

hydrated than the red slate and in places has a soft, clayey consistency. 
Siliceous phases of the slates which show gradation into ferruginous 
chert occur also. These are thinly laminated, but are hard and in places 
contain thin chert layers. The ferruginous slates represent stages in the 
deposition of the iron-bearing rocks when fine, argillaceous sediment 
rather than silica was being deposited with the iron. They are generally 
not slates in the proper sense of the word, as they rarely show slaty 
cleavage but are rather indurated, ferruginous shales or ferruginous 
argillites. They are not as abundant in the Cuyuna district as in some 
of the other Lake Superior iron-ore districts, especially in the Mesabi. 

The ferruginous slates differ from the hematitic schist, described on 
page 119, in that they are sedimentary, having been deposited at the same 
time as the ferruginous cherts, and having suffered the same alterations. 
The hematitic schist on the other hand results from the alteration and 
impregnation with iron oxide of the green chloritic schist, a rock of prob- 
able igneous origin, which is commonly associated with the iron-bearing 
rock. Most of the ferruginous slates cleave parallel to the original 
bedding laminations ; hematitic schist cleaves parallel to the schistosity. 
Certain ferruginous slates, however, which are derived from the oxidation 
of green amphibolitic slates also have well-developed secondary cleavage. 

Cherty and argillaceous ferrous carbonates are believed to be the 
original rocks from which the iron-bearing beds of the Cuyuna range 
in their various phases are largely derived. Such rocks have been re- 
ported from many parts of the district, having been found in the deep 
drilling, and also in the deeper workings of some of the mines. Several 
different types occur with gradational phases between them. They van- 
in composition and texture. Some have a banded, laminated appearance 
while others are massive. Nearly all of them are dense and fine grained. 
They range from light gray to dark green or black according to the asso- 
ciated minerals present. Usually they consist of interlayered chert or 
argillaceous material with amphibole and iron carbonate. Magnetite is a 
common constituent, and with increasing magnetite and recrystallization 
of the other constituents these rocks grade into amphibole-magnetite rock 
and magnetitic slate. 

The banding of the cherty and argillaceous ferrous carbonate is largelv 
due to interlayering of different constituents. The laminae of chert 
and siderite are usually light gray or greenish, while those containing 
argillaceous material or amphibole are darker and with increasing mag- 
netite become almost black. Some phases of iron carbonate rock are 
light gray and consist almost entirely of chert and siderite which may 
be interlayered or may be irregularly intermixed. Grains of magnetite often 
occur along lamination planes, making the banding more marked. Other 
phases of iron carbonate rock contain considerable amphibole, and these 



THE CUYUNA IRON-ORE DISTRICT 



117 



show interbanding of light gray and somewhat darker green layers. Still 
other phases are very dark gray or green and consist mainly of amphibole 
and argillaceous material, mixed with more or less siderite but containing 
little or no chert. Such phases are usually very fine-grained and show 
no marked lamination. All these varieties of iron carbonate rock are 
interbedded with each other and grade into each other. 

The siliceous magnetitic slate and amphibole-magnetite rock are phases 
of the original iron-bearing formation which have suffered metamorphic 
alteration and recrystallization. They are banded or laminated rocks 
usually very dark colored with a tinge of green. They consist mainly of 
amphibole and magnetite associated with chert or quartz. The fine- 
grained, finely laminated types with slaty structures are known as mag- 
netitic slates, while the more coarse-grained and coarsely-layered types 
are known as amphibole-magnetite rock. As is the case with ferruginous 
slate, magnetitic slate rarely, if ever, shows slaty cleavage and might per- 
haps be more properly designated magnetitic argillite. The banding in 
these rocks is due to the segregation of the different minerals into layers 
and also to a difference in the coarseness of crystallization. Thus layers 
of fairly pure magnetite alternate with layers of amphibole or of quartz 
and amphibole. Some bands are very fine-grained, especially those con- 
sisting mainly of magnetite, while other bands as those of amphibole are 
often medium coarse-grained. The segregation of minerals in the amphi- 
bole-magnetite rocks and magnetitic slates is never perfect. Small amounts 
of magnetite are usually disseminated through quartz and amphibole 
layers, while quartz, and amphibole occur intermixed with magnetite along 
the borders of magnetite bands. 

There is considerable variation in the composition of these rocks. 
Some consist largely of magnetite and amphibole and others contain a 
considerable amount of silica, either as chert or quartz, with amphibole, 
but with little magnetite. There is also some irregularity in the texture, 
such as the thickening or pinching out of laminae. These irregularities, 
however, are much less marked than they are in the ferruginous cherts. 

The amphibole-magnetite rock and magnetitic slate of the Cuyuna 
district differ from the typical amphibole-magnetite rock of some of the 
other Lake Superior iron-ore districts, such as the Mesabi, Gogebic, and 
Marquette districts. These rocks in the Cuyuna range are fine-grained, 
more perfectly laminated, and in general they contain less quartz and 
more ferromagnesian minerals. The typical amphibole-magnetite rock of 
the Cuyuna district is a finely banded rock consisting of alternating bands 
of magnetite and amphibole with a minor amount of quartz. That of the 
eastern part of the Mesabi district, on the other hand, is an irregularly 
banded rock consisting of alternating layers of fine-grained magnetite 



n8 GEOLOGY OF EAST CENTRAL MINNESOTA 

and coarse-grained quartz and amphibole. In the Cuyuna district, quartz- 
ose and cherty phases of the amphibole-magnetite rock and magnetitic 
slate are also common, however, and are characteristic, more especially, of 
less highly magnetitic phases of these rocks. 

With decrease of magnetite and iron carbonate, the slaty iron carbon- 
ate rocks and magnetitic slates grade into green amphibolitic slates. These 
are dark green rocks, nearly always finely and regularly laminated and 
with well-developed slaty cleavage at varying angles to the lamination. 
They consist mainly of a network of finely crystalline amphibole, but 
usually considerable iron carbonate is present and locally magnetite as 
well. Green amphibolitic slate occurs interbedded with other iron-bearing 
formation rocks or in areas adjacent to them. 

Jaspilite is of rather rare occurrence in the Cuyuna district, but well 
defined beds of it are found in several of the mines. Thus at the Cuyuna- 
Mille Lacs mine both the hard red-banded jasper and the specular, schis- 
tose jasper occur along the south wall of the north ore body, where they 
are interlayered between typical banded hematitic chert on one side and 
lean manganiferous iron ore on the other side. . 

Jaspilite is metamorphosed and recrystallized ferruginous chert. 
Where the metamorphism has not been very intense a hard, dense rock, 
bright red to reddish black, is formed. This is the more common phase, 
while the crystalline, specular form which results from pronounced re- 
crystallization is more local in its occurrence. Some of the hard, red 
jaspilite shows oolitic texture. In places in the Cuyuna district there is 
a very hard, dense, siliceous, ferruginous chert to which the term jasper 
is sometimes applied. This rock, however, varies in color from brown 
or reddish brown to black, and is simply a very siliceous ferruginous 
chert. 

The iron ore of the Cuyuna district is associated with different 
phases of the iron-bearing rock, varying in color, texture, and composi- 
tion according to the type of iron-bearing rock with which it is asso- 
ciated. Seven or eight main belts of the iron-bearing rocks run in a 
northeast-southwest direction through the district, and bodies of 
iron ore or manganiferous iron ore occur at intervals along nearly 
all of these belts. Both hard and soft ores are found, locally one 
being more abundant and elsewhere the other. They have suffered vary- 
ing degrees of hydration. Some consist of typical blue or reddish blue 
hematite, while others consist of yellow, ocherous limonite. All gradations 
between these two extremes are found. Some of the rich hematite ore 
like that at the Armour No. 2 mine is finely crystalline, but most of the 
ores and especially the more hydrated forms are amorphous. Locally 
fibrous, botryoidal limonite or goethite occurs in veins and geodes, but 
does not constitute an important ore. 



PLATE XVIII 




B. MINOR PLICATIONS IN THE IRON-BEARING ROCKS AT THE WEST END OF THE 

ROWE OPEN PIT- 



THE CUYUNA IRON-ORE DISTRICT 



ii9 



The manganiferous iron ore consists mainly of some form of iron 
oxide mixed with manganite, pyrolusite, or wad. In most of the man- 
ganiferous iron ore bodies, local masses of pure black manganese ore 
occur but are usually present in minor quantity and are irregularly inter- 
mixed with manganiferous iron ore or manganiferous iron-bearing rock. 
The manganiferous iron ores vary in color from dark red or brown to 
black, and are usually fine-grained to finely crystalline in texture. In a 
few places vugs of needle ore consisting either of pyrolusite or man- 
ganite have been encountered. 

The green and gray chloritic schist, commonly known as green schist, 
and the dark red hematitic schist do not belong to the iron-bearing rocks 
proper, but are closely associated with them. They generally occur as small 
lenses within the iron-bearing layers, or they may bound such layers on 
either side. The dark red hematitic schist usually occurs along the bor- 
ders of the lenses where the schist is in contact with the iron-bearing rock 
or iron ore and has become impregnated with iron oxide for varying dis- 
tances from the contact, while the green or gray chloritic schist occurs 
in the center of such lenses, or forms entire layers where they are not in 
contact with iron-bearing rock. 

As will be mentioned later, it is believed that the chloritic schists repre- 
sent original masses of subsilicic igneous rock which were intruded as 
sills into the iron-bearing rock and associated sedimentary rocks, while 
these were still largely in an unmetamorphosed condition. Some of it 
may be of volcanic origin, having been formed as local irregular flows, 
or as beds of ash or agglomerate. Subsequent metamorphism and de- 
formation were suffered alike by igneous rocks and sediments. 

Analyses have been made for the writers by the School of Mines 
Experiment Station, University of Minnesota, of type specimens of dif- 
ferent phases of the iron-bearing and associated rocks with the view of 
comparing the composition of the different varieties of rock, as well as 
of determining approximately the range in composition of some varieties. 
These are given in the accompanying table. 

Schist, slate, quartzite, and associated igneous rocks. — The beds of 
iron-bearing rock practically throughout the Cuyuna district, are inclosed 
between layers of schist or slate. Generally the schist or slate on one 
side of the iron-bearing layer is identical with that on the other side. 
Locally, however, the rocks forming the two walls present marked dif - 
ferences, but such differences are rarely constant over large areas. 

A number of distinct types of schist and slate are found in the Cuyuna 
district, among which are (i) white, greenish or brownish, light-colored, 
quartzose, sericitic schist or slate, (2) gray, finely micaceous schist, phyl- 
lite, or slate, (3) dark green, laminated, amphibolitic slate, (4) dark 
green or gray chloritic schist, commonly micaceous and locally quartzose, 



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122 



GEOLOGY OF EAST CENTRAL MINNESOTA 



as already described in connection with the iron-bearing rocki, (5) dark 
red hematitic schist derived from green chloritic schist by impregnation 
with hematite, (6) dark red hematitic slate derived by oxidation or im- 
pregnation with hematite from amphibolitic slate, and (7) black carbon- 
aceous slate, generally containing more or less pyrite. The distribution 
of these various schists and slates is irregular, but some are more abun- 
dant in one locality and others elsewhere. Thus quartzose sericitic schist 
is especially abundant in the region lying south of the north range, chlo- 
ritic schist is very common in association with the iron-bearing belts 
running from Ri vert on northeastward to Crosby and Cuyuna, while black 
carbonaceous slate is conspicuous locally in the northeastern portion of 
the south range and in the northern part of the north range. 

Although most of the slate and schist of the Cuyuna district is un- 
doubtedly of sedimentary origin, there are certain types, as the green 
chloritic schist, which in their occurrence and relation to the associated 
rocks strongly suggest an igneous origin. The chloritic schist generally 
occurs in comparatively short thick lenses, which are interlay ered with 
other rocks without any apparent order or regularity, although they gen- 
erally lie parallel to the bedding of the enclosing rocks. The chloritic 
schist rarely shows lamination or other sedimentary characteristics, and 
its contact with the associated rocks is sharp and without gradational 
phases. Its chemical and mineral composition has not yet been studied 
in sufficient detail to allow its definite classification, but indications point 
toward an igneous origin. Much of the light-colored, quartzose, sericitic 
schist or slate is siliceous, and local phases of it might be termed schistose, 
sericitic quartzite. The gray phyllite on the other hand is usually very 
fine-grained and silky in texture and rarely contains much quartz. The 
green amphibolitic slate consists mainly of a network of finely fibrous 
amphibole, but contains a considerable percentage of carbonate minerals. 
This rock is closely associated with the manganiferous iron-bearing rocks 
north and south of Menomin Lake. 

Quartzite has been reported as occurring in several parts of the dis- 
trict, notably in the northeastern portion of the south range and near the 
southwest end of the north range. In the latter locality, beds of typically 
fine-grained, vitreous quartzite have been encountered in drilling opera- 
tions along a belt more than a mile long, beyond which it seems to dis- 
appear in the schist areas. Locally iron-bearing rock and black slate 
are associated with it, but in general chloritic and micaceous schists occur 
on both sides of the quartzite belt. In the northeastern part of the south 
range also quartzite has been found by drilling in a number of localities, 
especially in the neighborhood of Cedar Lake. About 10 or 12 miles 
east of this region are the quartzite outcrops at Dam Lake. Some con- 
nection may exist between the rocks of the two regions. 



THE CUYUNA IRON-ORE DISTRICT 



123 



Igneous rocks, in which the original constituents have been largely 
altered and which have suffered considerable deformation locally, have 
been encountered in drilling operations in different parts of the district. 
Most of them seem to have been originally of a subsilicic character and 
now consist mainly of chlorite. Some still show the original igneous 
textures, while others have been rendered schistose and grade into chlo- 
ritic schist. Gradations between the schistose and non-schistose phases 
have been found locally. 

These rocks are believed to be of approximately the same age as the 
associated metamorphosed sediments, namely upper Huronian. The less 
schistose phases are usually medium to fine-grained. When compara- 
tively fresh they are difficult to distinguish from the later igneous intru- 
sives which are believed to be of Keweenawan age. Of the many bodies 
of igneous rock found in the district, very few except the schistose types 
can be definitely classified with the earlier or the later group. The rela- 
tions can be determined only by a careful study of the shape of the igneous 
rock masses, of the degree of metamorphism which they have suffered, 
and of their relation to the enclosing rocks. 

The form of the masses of igneous rock associated with the meta- 
morphosed sediments has rarely been determined by the drilling, although 
a few of the masses have been found to be irregularly interlayered with 
rocks of sedimentary origin. Probably many of them represent intrusions, 
but some have textures that resemble those of extrusive igneous rocks. 
In general, however, the alteration has progressed to such an extent that 
the original character is not determinable from the texture. 

LATER INTRUSIVE ROCKS 

Later intrusive rocks which are believed to be of Keweenawan age 
and which have themselves suffered little or no deformation or dynamic 
metamorphism, are known to occur in many parts of the district. They 
are practically all of the type usually classed as greenstones, including 
diorite, diabase, gabbro, and perhaps even less silicic varieties. Most of 
them are fine-grained dark green rocks of granular or ophitic texture 
such as occur at the Barrows mine. At one or two localities, as at the 
Adams mine, a porphyritic rock has been encountered. Many of these 
intrusive rocks have been altered so that at present they consist mainly 
of chlorite and decomposed feldspar. Some, however, are still compara- 
tively fresh, having suffered weathering only for a short distance down- 
ward from the lower surface of the glacial drift. 

The shape of most of the intrusive bodies is not known, as drilling 
has been conducted so as to avoid them rather than to outline them. 110 
Some are known to be in the form of dikes, others are irregular. 



"^Zapffe, Carl, The Cuyuna iron-ore district of Minnesota: Supplement to the Brainerd 
Tribune, July 1, 191 1. 



124 GEOLOGY OF EAST CENTRAL MINNESOTA 

The metamorphic effect of the intrusions along their contact with 
the older rock is in most places clearly noticeable. Where the igneous 
rocks have been intruded into the original iron-bearing rocks, they have 
usually caused a recrystallization. Chert is changed to quartz, and lime, 
magnesia, and other constituents are changed to amphibole. The iron 
oxide is in many places altered to magnetite. Thus typical magnetitic 
slate and amphibole-magnetite rock have been developed under the action 
of intrusions. Whether or not most of the rocks of this type in the district 
have been produced in this manner, or whether they were more generally 
formed during the deformation is not known. Doubtless the intrusions 
were locally effective, but from present indications they do not appear to 
have been sufficiently wide-spread to account for much of the meta- 
morphism over large areas. 

LATER VOLCANIC AND SEDIMENTARY ROCKS 

Volcanic rocks which lie on the eroded surface of the older rocks are 
reported by Van Hise and Leith 111 to occur at several localities a short 
distance south and southwest of Brainerd and also about six miles east 
of Brainerd. They are said to be silicic and to have amygdaloidal texture. 
The beds range up to 15 or 25 feet thick and seem to occupy depressions 
in the underlying rock surface. 112 These volcanic rocks are believed to 
be of Keweenawan age. 

Isolated patches of sediments lying in horizontal beds on the eroded 
surface of the iron-bearing rocks, schist, and associated rocks have been 
encountered in drilling and mining operations at a number of localities 
in the Cuyuna district. These rocks are believed to be of Cretaceous age 
because of their similarity to isolated patches of Cretaceous rocks in the 
Mesabi district. Cretaceous rock that bears a similar relation to the 
underlying metamorphosed rocks, also crops out at several points south- 
west of the district. In the Cuyuna district proper this rock is a ferru- 
ginous conglomerate, consisting of small pebbles of iron-bearing and 
other rocks in a slaty matrix. 113 Outside of the district the Cretaceous 
beds consist mainly of sandy and calcareous clay. 

OCCURRENCE OF THE IRON-BEARING ROCKS AND IRON ORE 
DISTRIBUTION OF IRON-BEARING ROCKS AND ORE 

As has been stated, seven or eight main belts of iron-bearing rocks 
trend approximately northeast-southwest through the district. Some of 
these are less than a mile long, as, for example, certain belts in the north 



m Van Hise, C. R., and Leith, C K., The geology of the Lake Superior region: U. S. 
Geol. Suney Mon. 52, p. 215, 1911. 
"'Zapffe, Carl, op. cit. 
,u Ibid. 



THE CUYUNA IRON-ORE DISTRICT 



125 



range and in Morrison County at the southwest end of the south range, 
while others as the main south range belt extend almost continuously for 
many miles. 

The most northerly known iron-bearing rocks in east central Minne- 
sota occur in the region of Lake Emily in T. 138N., R. 26W., about 35 
miles southwest of the west end of the Mesabi district. The extent of 
these iron-bearing rocks is as yet only imperfectly known. Farther south 
bands of low grade iron-bearing rocks are found in T. 136N., R. 26W., 
and T. 136N., R. 25W., a short distance north of Mississippi River, these 
being the northeasterly extensions of the northern iron-bearing belts 
in the north range. Besides these, a few occurrences of iron-bearing rocks 
are known in the region northwest of Aitkin in T. 137N., R. 25 W. 

The iron-bearing belts which have so far been found to contain ore 
bodies of commercial importance are practically confined to the area south 
and east of Mississippi River in Crow Wing County. However, some ore 
bodies have been found along the iron-bearing belts in Morrison County 
west of Mississippi River, although as yet no attempts have been made 
to develop them. The northerly iron-bearing belts south of Mississippi 
River are included in the north range and the southerly belts in the south 
range. 

The most northerly north range iron-bearing belt is only indefinitely 
known. Traces of it occur in section 25, T. 47N., R. 30W., and thence 
northeastward in sections 20 and 22, T. 47N., R. 29W., beyond which it 
crosses Mississippi River, as has been noted. No important ore bodies 
have been found along it. 

The next belt of iron-bearing rocks to the south is of considerable im- 
portance, containing the manganiferous iron-ore bodies of the Ferro and 
Algoma mines in sections 32 and 33, T. 47N., R. 29W., and other im- 
portant ore bodies in sections 28, 22, 27, and 23, T. 47N., R. 29W., the 
latter in the vicinity of Island Lake. Certain breaks occur in the con- 
tinuity of the belt, but in general it has been fairly well determined by 
extensive exploration. The iron-bearing rocks appear to be more or less 
manganiferous throughout. 

South of this belt of manganiferous iron-bearing rock and ore, there 
are a number of scattered occurrences of iron- and manganese-bearing 
rocks in the region north and west of Menomin Lake and southwest of 
Rabbit Lake, such as those in sections 34 and 35, T. 47N., R. 29W. The 
relation between the separate areas is not yet known. They extend south- 
westward as far as section 12, T. 46N., R. 30W. To the northeast of them, 
south of Rabbit Lake, is the important iron-bearing belt containing the 
Kennedy ore bodies. The Kennedy belt is known from the southern 
part of section 25, T. 47N., R. 29W., as far east as the eastern part of 



126 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Rabbit Lake where it either dies out or turns to the south and doubles 
back on itself. 

A short distance south of the eastern end of the Kennedy iron-bearing 
belt, is the northeastern end of another belt of iron-bearing rocks which 
lies to the south. It is uncertain whether these two belts are actually con- 
nected. Possibly they form the limbs of a syncline. The southern belt 
runs southwestward through the northwestern part of section 32 and the 
centers of section 31, T. 47N., R. 28W., and section 1, T. 46N., R. 29W., 
into the broad belt of iron- and manganese-bearing formation lying north 
Mid northwest of Ironton and Crosby in sections 2, 11, and 10, T. 46N., 
R. 29W., which contain the Mahnomen and Mangan No. 2 ore bodies 
along the northern margin and the Thompson north ore body and the 
Armour No. 1 and Pennington ore bodies along the southern margin. 
West of the Mahnomen and Pennington mines the belt divides into two 
parts separated by a strip of slate and schist. The northern part has been 
explored for only a short distance southwestward into section 9, while the 
southern part extends southwestward through the Feigh property in sec- 
tion 10, and along the north side of Blackhoof Lake in the southern part 
of section 9 where it contains the Hillcrest and other ore bodies. Thence 
it continues through the northwestern corner of section 16 and the north- 
ern part of section 17 into section 18, T. 46N., R. 29W "., where the Rowe 
mine is located. Here it makes a synclinal turn northward and doubles 
back on itself for a short distance. The extent of the backward turn is 
not yet known. 

This iron- and manganese-bearing belt is one of the most important in 
the district. It is known continuously over a length of more than 8 miles, 
and along it are some of the most important ore deposits in the district. 
Unlike the belts to the north, this belt contains many important iron-ore 
deposits, but manganiferous iron ores are found in it at various places also. 
On the north it is separated along the central portion, from the Cuyuna- 
Mille Lacs-Sultana manganiferous iron-bearing area by a thin band of 
slate or schist, while on the south another thin band of schist separates 
it from the important Croft-Armour No. 2 iron-bearing belt. 

The Cuyuna-Mille Lacs-Sultana manganiferous iron-bearing area is 
in section 3, south and southeast of Menomin Lake. It is manganiferous 
throughout and contains important bodies of manganiferous iron ore. 
It is known to be quite wide where the Sultana and Cuyuna-Mille Lacs 
mines occur, but its longitudinal extent northeast and southwest is only 
imperfectly known. Areas of iron-bearing formation, however, occur 
north and south of June Lake in the northern part of section 9, and these 
may have some connection with the Cuyuna-Mille Lacs-Sultana area. 
Other ore bodies occurring in the Cuyuna-Mille Lacs-Sultana ore-bearing 



THE CUYUNA IRON-ORE DISTRICT 



127 



area are those of the Mangan No. 1 and the Hopkins mines, the latter 
containing both iron ore and manganiferous iron ore. 

The Croft-Armour No. 2 belt has been well explored to the northeast 
as far as section 1, T. 46N., R. 29W., where the Croft mine occurs. From 
this point it runs southwestward through the Meacham, Thompson, Ar- 
mour No. 2, and Ironton properties in section 11, T. 46N., R. 29W., and 
continues a short distance into the eastern part of section 10, where it 
pinches out. The belt is fairly narrow throughout its length, but contains 
important iron ore bodies. North of the I ronton- Armour No. 2 ore body 
and the Thompson south ore body which occupy its central portion in 
section 11, is a band of green schist which separates it from the ore body 
of the Pennington and Armour No.. 1 mines. 

The Croft-Armour No. 2 belt consists very largely of iron-bearing 
formation and associated iron ore, though locally, as in the eastern part 
of the Armour No. 2 property and the northern part of the Thompson 
south ore body manganiferous iron-bearing formation and ore occur in 
it. The proportion of ore to iron-bearing rock in the belt is high, the ore 
bodies consisting largely of a somewhat siliceous, hydrated, brown to red 
hematite, but in places containing large masses of red, medium soft, high- 
grade hematite, some of which is of Bessemer quality. The ore-bearing 
belt is bounded on the south mainly by green chloritic schist and red 
hematitic schist. 

South of the Croft-Armour No. 2 and the Pennington-Rowe iron- 
bearing belts, are local occurrences of manganiferous and non-mangan- 
iferous' iron-bearing formation and ore, such as those in sections 19, 17, 
12, and 1, T. 46N., R. 29W. These are the southernmost areas of iron- 
bearing formation in the north range. 

The south range belts of iron-bearing rocks, although narrow, are 
far more extensive longitudinally than the iron-bearing belts of the north 
range. The most northeasterly occurrences of south range iron-bearing 
rock are in the Rice River region northeast of Aitkin. From this point 
southwestward as far as Deerwood a number of areas of iron-bearing 
rock are known. They are approximately along the same strike, but there 
are large unexplored areas between them. 

From Deerwood the south range belt of iron-bearing rocks has 
been traced for about 24 miles more or less continuously southwestward 
to a point a short distance southwest of Barrows. Mines and exploratory 
shafts occur along it at intervals where ore bodies have been found, the 
Adams mine being located in section 30, T. 46N., R. 28W., the Hobart 
Iron Co. (of Pickands, Mather and Company) shaft in section 8, T. 45N., 
R. 29W., the Wilcox mine, near Woodrow, in section 13, T. 45N., R. 
30W., the Adbar Development Company shaft in section 22, T. 45N., 
R. 30W., the Brainerd-Cuyiina mine, near Brainerd, in section 36, T. 45N., 



ij6 GEOLOGY OF EAST CENTRAL MINNESOTA 

R. 31W., the Barrows mine, near Barrows, in section 10, T. 44N., R. 
31W., and the Rowley shaft, also near Barrows, in section 16, T. 44N., 
R. 31W. Besides the more or less developed ore bodies on which mines 
and exploratory shafts are located, there are numerous other ore bodies of 
importance at various points along the belt, as shown by drilling. 

Throughout most of its extent, the main south range beh of iron- 
bearing rocks occurs as a single band. There are, however, small stretches 
containing two or three parallel bands. In places these are due to a break- 
in a band where the ends are pushed past each other as in a drag fold 
or thrust fault ; elsewhere the repetition has probably resulted from 
extensive folding. There are also numerous places along the belt where 
iron-bearing rock has not yet been found, and barren stretches occur 
which apparently break the continuity of the iron-bearing belt. Future 
exploration may result in the discovery of iron-bearing formation in 
such areas. 

South and southeast of Brainerd and south of Barrows, a second belt 
of iron-bearing rock trends parallel to and lies about \ l / 2 miles south of 
the main south range belt. Ore bodies have been found along it here and 
there, but it is not yet thoroughly explored. 

The Morrison County belts of iron-bearing rock are the continuations 
of the south range belts west of Mississippi River. Most of them are 
short. They lie approximately parallel to each other and cover an ex- 
tensive area in which ore bodies have been found at a number of places. 

Isolated bands of iron-bearing rock have been found at several locali- 
ties south of the south range in Crow Wing County. Among the most 
important of these is one which contains a considerable amount of iron 
ore mixed with ferruginous chert, in sections 8 and 9, T. 45N., R. 28W.. 
about halfway between Clearwater Lake and Bay Lake. Another isolated 
band has been found near Clear Lake in the southern part of T. 46N.. 
R. 25W., and the northern part of T. 45N., R. 25W., in Aitkin County. 

The areas between the different belts of iron-bearing rock have been 
only very imperfectly explored. Practically wherever drilling has been 
done, however, some variety of schist or slate has been encountered, such 
as chloritic, amphibolitic, sericitic, quartzose, and graphitic or carbona- 
ceous schist, or slate. Green and gray micaceous and chloritic schist appear 
to be by far the most abundant. Locally quartzite has been encountered, 
but usually it is in small amounts, occurring apparently in local lenses. 
Elsewhere considerable areas of subsilicic igneous rocks have been found. 

It is not impossible that as exploration work progresses other areas 
of iron-bearing rock and ore may be encountered between the belts now- 
known. Drilling has up to the present been confined largely to areas 
showing abnormal magnetic attraction. Nevertheless iron-bearing rock 
and ore have been found in several places where no marked abnormal 



THE CUYUNA IRON-ORE DISTRICT 



129 



magnetic attraction existed. It seems possible that as the drilling work 
is extended over larger areas, iron-bearing rock may be found where it 
is hitherto unsuspected. 

The iron-ore bodies are found at intervals along the belts ot iron- 
bearing rock. In some belts they are very abundant, while in other belts 
they are almost absent. Even along single belts of iron-bearing rock 
they are more or less grouped in certain parts with stretches of barren 
iron-bearing rock between. Thus along the Armour No. 2-Thompson 
belts, ore is almost continuous, while in the Armour No. 1 -Pennington 
belt important ore bodies are grouped together in some parts, and else- 
where they are almost wanting. Other belts, as the main south range 
belt, have ore bodies scattered along them at irregular intervals with bar- 
ren areas consisting of ferruginous chert, ferruginous slate, amphibole- 
magnetite rock, or magnetitic slate between. 

OCCURRENCE AND CHARACTER OF ORE 

As already stated, the iron-bearing beds are composed of a number 
of different kinds of rocks. The different varieties vary in abundance in 
different parts of the district, as well as in different parts of the same belt. 
The most abundant and most generally distributed of the iron-bearing 
rocks is the ferruginous chert, and it is with this rock that the iron ore 
is commonly associated. Ferruginous slate also is abundant, but is not 
as commonly associated with iron ore. Along the manganiferous iron-bear- 
ing belts, however, manganiferous iron ore bodies are frequently found in 
ferruginous slate as disseminated masses and irregular impregnations. 
The mining development in the district is not as yet sufficiently advanced 
to give more than a general idea of the occurrence of the other phases 
of the iron-bearing rock such as the cherty or argillaceous iron carbonate 
rock and amphibole-magnetite rock. Iron carbonate rock is usually found 
at a greater or less depth below other iron-bearing rocks, while amphibole- 
magnetite rock occurs where metamorphism of the original iron-bearing 
rock has taken place. Amphibole-magnetite rock is frequently associated 
with ore, and there is good evidence that certain types of iron ore are 
closely related with it in origin. 

The ferruginous chert occurring in the Cuyuna district is very similar 
to that found in most of the other Lake Superior iron-ore districts ex- 
cept the Mesabi. In all the Michigan and Wisconsin districts the com- 
mon rock composing the iron-bearing formation is a more or less regu- 
larly banded ferruginous chert, believed by Van Hise and Leith 114 to 
have been derived by alteration from an original cherty iron carbonate 
rock. In the Mesabi district the ferruginous chert is largely of the type 



u «Van Hise, C. R., and Leith, C. K., op. cit., pp. 499-5 60 



GEOLOGY OF EAST CENTRAL MINNESOTA 



known as taconite, a rather massive, bedded cherty rock irregularly 
speckled with iron oxide which is believed to have resulted from the 
oxidation of the ferrous silicate rock, greenalite rock. No taconite has 
been found in the Cuyuna district, nor has any greenalite been encoun- 
tered. It is, therefore, believed that here as in most of the other dis- 
tricts, the original rock from which the present hematitic and limonitic 
chert and iron ore have in large part been derived is a banded cherty fer- 
rous carbonate rock. 

It is not the object in this paper to present a discussion on the origin 
of the iron-bearing rocks and ores. This is reserved for the final report 
for which more detailed information is being gathered. In view of a 
recent tendency, however, to question the hypothesis of geologists 
expressed in earlier reports that most of the iron-bearing formations of 
the Lake Superior region were deposited originally as ferrous carbonate 
or silicate, and to supplant it by the hypothesis that the iron was de- 
posited in the ferric form originally, it may be interesting to mention 
some occurrences in the Cuyuna district which have a bearing on this 
subject. 

Leaving out of consideration the source of the iron, it may be briefly 
stated that the older hypothesis supposes the iron to have been deposited 
from the original solutions largely under reducing conditions as ferrous 
carbonate or ferrous silicate in association with colloidal silica or with 
argillaceous material. Locally and at certain stages in the deposition, 
ferric oxide also was probably precipitated, either in a relatively pure 
form or along with colloidal silica or argillaceous material. However, 
the ferric oxide deposition is believed to be of relatively minor importance. 
Eventually these sediments were raised into the belt of weathering and 
were transformed by surface agencies into their present condition. The 
ferrous carbonate and ferrous silicate rocks were oxidized to ferruginous 
chert and ferruginous slate, and the ferruginous chert by the leaching 
of silica was altered to iron ore. Dynamic metamorphism previous to 
oxidation in some localities resulted in the alteration of the ferrous car- 
bonate or ferrous silicate to amphibole-magnetite rock, and the latter also 
was locally in part altered to ore by weathering processes where the 
previous metamorphism was not very intense. 

The other hypothesis differs from this in that the iron is supposed 
to have been deposited largely as ferric hydroxide, locally in a compara- 
tively pure form, but more generally along with impurities such as col- 
loidal silica or clay. In places the presence of carbonaceous matter or 
other deoxidizing agents probably resulted in a partial deoxidation of 
the ferric hydroxide and the formation of local masses of ferrous sedi- 
ments. Upon metamorphism, these materials were consolidated and in 
part dehydrated, forming the present ferruginous chert and slate with 



THE CUYUNA IRON-ORE DISTRICT 



131 



local original lenses of ore and bodies of ferrous carbonate or ferrous 
silicate. Later weathering and erosion processes resulted in the forma- 
tion of additional iron ore by the leaching of silica from the ferruginous 
chert,, and also in the oxidation of some of the ferrous carbonate and 
ferrous silicate originally present. 

The main difference between the two hypotheses is that according to 
the older one most of the iron-bearing rock was at one time largely in the 
ferrous state; while according to the later one small masses of ferrous 
sediments were only locally formed due to the presence in places of 
deoxidizing agents. 

The evidence afforded by the Cuyuna district on this question is largely 
in favor of the older view. In the exploratory drill work, ferrous car- 
bonate was encountered below a surface capping of ferruginous chert 
and iron ore, in many parts of the district, while more recently in some 
of the mining operations ferrous carbonate with associated amphibole- 
magnetite rock and magnetitic slate has been encountered in the lower 
levels directly continuous along the bedding with the ore and ferruginous 
chert of the upper levels. In general, the gradation from ore at the sur- 
face through lean ferruginous chert into ferrous rocks below is perhaps 
more typically illustrated in parts of the Cuyuna district than in any of 
the other Lake Superior iron-ore districts. 

The ferrous carbonate rock of this district, however, differs from the 
typical cherty iron carbonate rock which has been described from other 
districts. A number of different phases occur, some cherty, some argil- 
laceous, and others containing amphibole and magnetite. Apparently 
various gradations occur between ferrous carbonate rock and amphibole- 
magnetite rock. Their general character and lithology have already been 
described. Their occurrence is usually irregular. While occasionally 
magnetitic slate and amphibole-magnetite rock are found practically at 
bed rock surface, more generally they, as well as the ferrous carbonate 
rock, are found at some depth. In places, ferrous carbonate rock is also 
found as masses within the oxidized iron-bearing rocks. 

The drilling has not yet been deep enough or thorough enough to 
give any indication of the average depth at which ferrous rocks are en- 
countered below the oxidized iron-bearing formation phases. At the 
Kennedy mine, ferrous carbonate rock is abundant on the 262 foot level, 
while in some other parts of the district drilling has shown ferruginous 
chert continuing to a depth of 800 feet or more. The depth of oxidation 
is dependent both on the original nature of the material and on various 
structural conditions such as fracturing, folding, and faulting. 

The average depth to which the Cuyuna ore bodies may be expected 
to extend is also a doubtful question. It is possible that locally beds of 
ferruginous chert or ore occur which were originally deposited as such. 



132 



GEOLOGY OF EAST CENTRAL MINNESOTA 



and these may extend to great depths without much change in their char- 
acter. On the other hand, where the ferruginous chert was undoubtedly 
formed by the oxidation of original ferrous sediments and the ore has 
resulted from the leaching of silica from lean iron-bearing rock, the de- 
posits do not extend to great depths, except perhaps in a few instances 
where exceptionally favorable structures occur for secondary concen- 
tration. 

The Cuyuna iron-ore bodies are as a rule roughly tabular in shape, 
the longer axes being parallel to the bedding of the enclosing rocks. As 
the beds usually dip steeply, the ore bodies are shown at the surface as 
bands that extend for considerable distances along the strike of the beds. 
They vary in width up to several hundred feet, and some of the known 
ore bodies are more than a mile long. Their behavior with depth 
varies. Some are comparatively shallow, extending perhaps to a 
depth of ioo or 200 feet below bed rock surface, where they 
give way to ferruginous chert, amphibole-magnetite rock, or unaltered 
iron-bearing rock. Others extend to greater depths, either continuing 
directly downward along the bedding, or running diagonally downward 
parallel to the strike in the form of shoots. The data regarding the 
depths reached by the ore bodies is still very incomplete. The deepest 
mining operations at present extend to a depth of about 280 feet below 
the base of the glacial drift, and to this depth ore is known to continue 
uninterruptedly. The exploratory drill work in the district has for the 
most part been shallow, few holes occurring which reach a depth greater 
than 700 feet. Ore, however, has been encountered in some of the deeper 
holes, and it is suspected that some ore bodies may reach a considerable 
depth. 

The ore bodies are usually enclosed between walls of ferruginous 
chert, ferruginous slate, or other iron-bearing rocks. Where ferruginous 
chert forms the wall rock, the contact is usually very irregular. Beds 
and irregular horses of ferruginous slate and chert are of frequent oc- 
currence in the ore bodies. Green chloritic schist and dark red hematitic 
schist are also common associates of the iron ore. They may form the 
wall rock of the ore bodies, or they may occur interlayered with the ore, 
more or less parallel to the bedding. Other rocks associated with the 
ore bodies are amphibole-magnetite rock, magnetitic slate, ferrous car- 
bonate rock, green and gray sericitic and siliceous slates, and black car- 
bonaceous slate. These are especially common along the south range, 
where they form the wall rocks in many places. In the north range, the 
ferrous rocks in places occur underneath ore bodies representing the 
unaltered portions of the iron-bearing formation. 

Most of the iron ore occurring in the ore bodies of the Cuyuna dis- 
trict is soft, but hard ore is also abundant both in the north and south 



THE CUYUNA IRON-ORE DISTRICT 



133 



ranges, and is more or less irregularly associated with the soft ore. The 
Cuyuna ore probably shows a greater variety of texture, composition, and 
color than the ore from any of the other Lake Superior iron-ore districts. 
It shows all stages of hydration from pure reddish blue hematite to 
ocherous, yellow limonite, and both argillaceous and siliceous phases are 
common. On the north range the ores vary from dark reddish blue, high- 
grade hematite, to reddish and yellowish brown siliceous or argillaceous 
hematite and limonite, while siliceous and argillaceous black, red, and 
brown manganiferous iron ore is common. Locally wash ore is asso- 
ciated with the other ores. In the south range, the typical ores are red- 
dish brown hydrous hematite, dark brown to black limonite, and yellow, 
ocherous limonite. 

The richest ore that has been found in the district is a medium soft, 
reddish blue hematite which occurs at the Croft mine and locally in the 
Armour No. i, Armour No. 2, Ironton, and Pennington mines in the 
north range. Much of this ore, especially that at the Croft mine, is of 
Bessemer quality, samples from the latter deposit analyzing as high as 
67 to 68 per cent metallic iron and as low as .01 to .03 per cent phos- 
phorus. While ore of this grade is, of course, exceptional, a large part 
of the Croft ore body consists of ore of Bessemer quality and will be 
mined as such. In the other mines no attempt is made to mine the Bes- 
semer ore separately. In the Armour No. 2 and Ironton mines, much of 
this type of ore is finely crystalline. 

The most common ore in the north range is a soft to medium hard, 
brown to reddish or bluish brown, hydrated hematite and limonite. This 
ore is locally highly siliceous, and grades with increasing silica into fer- 
ruginous chert and wash ore. Much of it also is somewhat argillaceous. 
Ore of this type is found at the Kennedy, Thompson, Armour No. 1, 
Armour No. 2, Pennington, and Rowe mines. It usually occurs in some- 
what irregular, lenticular bodies of different sizes which are enclosed in 
or associated with hematitic and limonitic chert and slate. Often ferru- 
ginous chert is interbedded with the ore, or occurs in the ore bodies as 
irregular masses. The character of the ore varies somewhat with the 
nature of the enclosing rock. Where the wall rock is mainly hematitic slate, 
as is the case in some of the deposits in the north range, the ore is soft, 
dark red or bluish red, and usually somewhat argillaceous. On the other 
hand, where ore is enclosed in ferruginous chert it is medium hard, some- 
what siliceous, and generally much fractured, so that it breaks into small 
blocks. Such ore usually ranges in grade from 58 or 60 per cent metallic 
iron downward, depending upon the amount of silica or alumina present 
The phosphorus ranges from .05 per cent up, and is frequently as high 
as .4 or .5 per cent. It is generally above .1 per cent. 



134 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Another variety of ore found in the north range is the finely lami- 
nated, brownish yellow limonite of the Mahnomen open pit. This ore, 
while usually relatively low in silica, is more hydrated than the other 
north range ores. The yellow ocherous limonite ore also is common in 
the south range, where its occurrence is similar to that of the black 
limonite ore. Usually it is soft and friable. While occasionally argil- 
laceous, as a rule it is quite pure, and ranges up to 56 or 57 per cent 
in metallic iron, which is nearly the theoretical limit for limonite. Com- 
bined moisture in the yellow ocherous ore may run as high as 14 or 15 
per cent. 

The so-called wash ore of the Cuyuna district has attracted con- 
siderable attention, and two washing plants have been erected to treat 
it. This ore is simply disintegrated ferruginous chert in which the chert 
is in the form of extremely fine sand, and can readily be washed out, 
leaving the iron oxide particles behind. The metallic iron content of the 
ore may be increased in this manner from 8 to 12 per cent. 

The soft black limonite ore is common on the south range, but is rare 
on the north range. It is generally associated with amphibole-magnetite 
rock and magnetitic slate. As a rule it contains considerable carbonate 
and is slightly magnetic, owing to disseminated particles of magnetite 
which occur in it as well as in other south range ores. This magnetite is 
probably residual from original magnetite-bearing rock which has been 
altered to ore by the leaching of soluble constituents. 

Manganese ore and manganiferous iron ore are common in the north 
range, especially in the northern part. All gradations occur, from an 
almost pure manganese ore with only a few per cent of iron, to iron ore 
with less than one per cent of manganese. The distribution of the dif- 
ferent grades of ore is very irregular. Xodules and small bodies of high- 
grade manganese ore usually occur scattered through low-grade man- 
ganiferous iron ore or manganiferous iron-bearing rock. They are too 
small to be mined separately but are mixed with the manganiferous iron 
ore. A number of manganese minerals occur in these deposits, chief 
among which are manganite. pyrolusite. and wad. Any of these minerals 
may compose the nodules or bodies of manganese ore. Often two or 
more of them are associated in the same body. 

The manganiferous iron ore is usually black, dark red, or dark brown. 
It occurs in irregular large or small bodies in ferruginous chert or fer- 
ruginous slate which may or may not be manganiferous. In places, the 
manganese-bearing portions of the iron-bearing rocks follow distinct 
zones more or less parallel to the bedding. The manganese content of 
the ore bodies varies greatly from place to place. The percentage of 
manganese commonly ranges from 1 or 2 per cent to 30 or 35 per cent. 



GEOLOGY OF THE PRINCIPAL MINES 



135 



Usually a decrease in the percentage of metallic manganese is accom- 
panied by an increase in the percentage of iron, the combined percentage 
of iron and manganese generally being fairly constant. As the combined 
percentage of iron and manganese decreases, silica and alumina Increase 
and the material becomes manganiferous iron-bearing formation. Man- 
ganiferous iron ore may be soft or hard. Some forms are dense and 
massive, while others are friable. It is uncertain which of the manganese 
minerals is associated with the hematite or limonite in the manganiferous 
iron ore. Possibly all the various manganese oxides are present in dif- 
ferent phases of the ore, but it is probable that manganite predominates 
in the hard ore and pyrolusite and wad in the soft ore. 

Iron-bearing rocks in which manganese ore and manganiferous iron 
ore occur in small and large bodies, are argillaceous or siliceous. Phases 
of it are very low in manganese, being simply ferruginous chert or fer- 
ruginous slate, while other phases are irregularly impregnated with small 
quantities of manganese oxide throughout, being a manganiferous iron- 
bearing rock. The bodies of manganese and manganiferous iron ore are 
found scattered irregularly through these manganese-bearing zones and 
differ in richness in different parts. As already mentioned the man- 
ganiferous iron-bearing rock commonly occupies more or less definite 
stratigraphic zones of varying width and length in the iron-bearing 
formation. 

The distribution of the manganiferous iron-bearing rocks in the 
Cuyuna district is somewhat irregular, although in general the more 
northerly iron-bearing belts are richer in manganese. The south range 
thus far has not shown the presence of much manganiferous material. 
Some iron-bearing belts as those on which the Armour No. 1, Armour 
No. 2, and Thompson mines are situated, have manganiferous iron-bearing 
rock on the foot wall while other belts such as that on which the Mah- 
nomen open pit is located contain manganiferous material in the hanging 
wall. In still other localities manganiferous iron-bearing rock and ore 
may be interbedded with iron-bearing beds. Thus there are all variations 
in the occurrence of the manganiferous material. 

GEOLOGY OF THE PRINCIPAL MINES 
NORTH RANGE MINES 115 

FERRO MINE 

The Ferro mine of the Onahman Iron Company, is situated in the 
SEJ4 of NE^, section 32, T. 47N., R. 29W., the shaft being in the 
foot-wall of the ore body which cuts in a northeast-southwest direction 



115 A few field notes made by H. G. Ferguson of the United States Geological Survey in 
July, 1917. have been inserted in the following descriptions, where indicated. 



136 



GEOLOGY OF EAST CENTRAL MINNESOTA 



across the southeast corner of the 40-acre tract, dipping steeply to the 
southeast. The property also includes several adjoining 40-acre tracts. 

The ore body in which the Ferro mine is located is near the south- 
western end of a belt of manganiferous iron ore and manganiferous iron- 
bearing formation which extends from the southern part of section 32, 
T. 47N., R. 29W., northeastward through sections 33, 28, 27, 22, and 23 
to about the center of section 24, T. 47N., R. 29W. In general the iron- 
bearing belt is of varying width, and is bounded on both sides by schist 
or slate, the nature of which varies at different places along the belt. The 
iron-bearing rock itself, where affected by surface weathering, consists 
mainly of ferruginous chert or manganiferous ferruginous chert with 
which is associated ferruginous slate, manganiferous iron ore and iron 
ore. With depth, much laminated dark green siliceous rock is encoun- 
tered which contains ferrous silicates and carbonate mixed with chert. 

The Ferro mine has a timber shaft 157 feet deep. The main hauling 
level is at 149 feet, and the working level from which ore is at present 
being mined is about 75 feet deep (summer, 191 7). The workings are all 
in iron-bearing rock and ore, no schist having as yet been encountered. 
The bottom of the shaft is in laminated, greenish black iron-bearing forma- 
tion which probably consists of a mixture of ferrous carbonate and sili- 
cates and chert, the so-called foot-wall of the ore body. A cross-cut to the 
southwest passes from this rock through the ore body and into dense 
black ferruginous chert, which forms the hanging wall. The glacial drift 
overlying the ore body is from 55 to 60 feet thick. 

The dip of the bedding of the iron-bearing rocks is from about 65 
southeast to vertical, and this is also the range in dip of the ore body. 
The latter occupies a certain zone in the iron-bearing beds, and appar- 
ently represents a replacement of the original rock along this zone by 
oxides of manganese and iron, chiefly manganese. This is well shown 
by the fact that the ore occurs along joints and minor cracks, and 
penetrates from these for varying distances into the mass of the rock. 
Many little rectangular blocks of ore are found which, on being broken, 
present an interior of decomposed or partially replaced rock. The nature 
of the original rock is not known. The character of the rock bounding 
the ore body leads one to suppose that it probably consisted largely of 
chert, ferrous carbonate, and ferrous silicate with perhaps considerable 
manganese carbonate and silicate. The decomposed residue, which at 
present acts as a sort of matrix in which the ore is embedded and in 
which it forms replacements, is a dark red, siliceous material rich in iron 
and usually soft and porous. Associated with the ore are certain layers 
of epidote rock, the relations of which are not yet known. 

The grade of the ore is variable. Masses of rich, finely crystalline 
pyrolusite occur locally, while elsewhere mixed manganese and iron 



GEOLOGY OF THE PRINCIPAL MINES 



137 



oxides are found. The matrix of dark red material in which the ore 
occurs can not be separated from it in mining, so that the ore as shipped 
ranges between 20 and 30 per cent in metallic manganese and about the 
same in iron. It is one of the best grades of manganiferous iron ore 
shipped from the Cuyuna district at the present time. 

The following is the average cargo analysis of ore shipped from the 
Ferro mine during the season 1916. 116 

Average Cargo Analysis of Ore Dried at 212 F. from the Ferro Mine 

Fe P Si0 2 Mn A1 2 0, 

Per cent 29.29 .075 17.56 22.29 1-47 

JOAN NO. 3 U7 

This is a new shaft mine, operated by the Joan Mining Company, 
situated in the NE>4 of SE*4 of section 32, T. 47N., R. 29W., about 500 
feet west-southwest of the Ferro mine, and apparently on the same ore 
body. Ore is at a depth of 95 feet. The strike is southwest and the 
dip is steep toward the north and vertical in places. Ferruginous slate, 
carrying a little carbonate, occurs on the south wall, and changes to 
limonitic material with bands of chert near the ore. The limonitic ore 
(partly replaced by manganese) is said to have a higher phosphorus con- 
tent than the more completely replaced black ore. 

ALGOMA MINE 

The Algoma mine of the Algoma Mining Company, is about one half 
mile northeast of the Ferro mine in the NE*4 of NW}4 of section 33, 
T. 47N., R. 29W. It is located on the same general iron-bearing belt 
as the latter, although there is apparently a slight break in it between the 
two properties. 

The main hoisting shaft is a lath shaft, which penetrates about 60 
feet of surface and about 100 feet of bed rock, while the timber shaft 
is about the same depth. The principal hauling level is no feet deep, and 
the main present working level is at about 80 feet (summer, 1917). There 
are several sublevels. As in the Ferro mine, all the workings are in iron- 
bearing rock and ore, no schist or slate having been encountered. The 
shaft is in the foot-wall rock, northwest of the ore body. 

The ore at the Algoma mine does not occur in a compact body, as 
that at the Ferro mine, but is found along four or five zones in the iron- 
bearing formation, running parallel to the bedding of the latter. The 
zones are separated by ferruginous chert which in some places is hard 
and siliceous, and elsewhere medium soft and decomposed. Along cer- 
tain layers it is highly manganiferous, while elsewhere it contains only 

ua Lake Superior Iron Ore Association, Analysis of Lake Superior iron ores, season 1916, 
P- 9. 1917. 

™ Ferguson. H. G. 



138 



GEOLOGY OF EAST CENTRAL MINNESOTA 



a trace of manganese. The ore zones vary in width from a few feet to 
20 feet or more. Some of them are continuous for a considerable dis- 
tance along the strike. 

The material varies in character in the different ore zones, as well as 
in different parts of the same zone. One zone consists mainly of ocher- 
ous, brown iron ore, speckled and blotched with impregnations of man- 
ganese ore. Others consist of uniformly black manganiferous iron ore, 
at some places soft and at others hard and siliceous. At one point a 
zone of manganiferous iron ore passes into high-grade iron ore along 
the strike. Thus there is considerable variation in the character as well 
as in the composition of the ore, and in order to maintain a uniform 
product, the ore from the different zones is mixed in mining. 

The general strike of the ore-bearing beds is perhaps a little more 
nearly north and south than at the Ferro mine. The dip is steeply south- 
east. Northeast of the Algoma mine the iron-bearing belt continues 
across the SEJ4 of section 28. where considerable drilling has 
been done, and where the MacKenzie mine is now being opened up. 
At this point the belt is wide and consists mainly of ferruginous chert, 
much of which is manganiferous and locally contains ore. To the north- 
west of it, green and gray schist have been encountered in drilling, and 
it seems probable that these rocks also bound the iron-bearing belt to 
the northwest in the Algoma and Ferro mines. Northeast of section 28, 
the belt has been explored along the line between sections 27 and 22, and 
also in section 23, south of Island Lake. At both of these localities it 
consists mainly of manganiferous iron ore and manganiferous iron-bearing 
rock. South of Island Lake, the belt is narrow and is highly manganifer- 
ous. It is bounded on the north by gray schist and on the south by gray 
schist and graphitic slate. 

The Algoma-Ferro iron-bearing belt, although varying in character in 
different parts, is distinctly manganiferous throughout its known extent. 
It varies greatly in width. At some places it is very narrow : in others 
it is 1,000 feet or more wide. It has a fairly uniform steep dip to the 
southeast or south. Its extent to the northeast and southwest beyond 
the present known limits is not yet thoroughly explored. Small areas of 
iron-bearing formation, however, are known at several localities beyond 
both ends. 

The following is the average cargo analysis of ore shipped from the 
Algoma (Tloch) mine during the season of 1916. 119 

Average Cargo Analysis of Ore Dried at 212 F. from the Algoma Mine 

Fe P SiO: Mn AI t O, 

Per cent 32.26 .090 16.47 1950 2.43 



"* Lake Superior Iron Ore Association, op. cit., p. 9. 



PLATE XIX 




GEOLOGY OF THE PRINCIPAL MINES 



no 



ROWE MINE 

The Rowe mine, operated by the Pittsburgh Steel Ore Company, is 
located in the N}4 of SEJ4 of section 18, T. 46N., R. 29W., on Little 
Rabbit Lake near Riverton, at the western end of the north range. The 
mine consists of a large open pit about 2,500 feet long, east and west, and 
about 700 feet wide from crest to crest at the widest part. The depth of 
the pit at the present time varies from about 100 feet on the north side 
near the center, to about 20 feet on the south side, the difference being 
due both to surface topography and to irregularity of excavation (sum- 
mer, 1917). The overburden, consisting mainly of sand and clay, varies 
in thickness from 15 feet at the west end of the pit to 74 feet on the north 
side. The difference in elevation of the ore surface underneath the over- 
burden of glacial till is about 30 feet, thus showing considerable irregu- 
larity of the pre-glacial surface over this comparatively small area. The 
greatest depth of the bottom of the pit below the bed rock surface is about 
60 feet, and is near the center of the pit. During the summer of 191 7 a 
concrete hoisting shaft was sunk south of the west end of the pit. 

Iron-bearing rock and ore are exposed in the pit over a length of 
about 1,900 feet and over a maximum width of 450 feet. The general 
strike of the iron-bearing belt is about N.63°E., but, owing to a sharp 
synclinal fold at the west end of the pit, the details of the structure 
within the pit are very complex, as may be seen from the strike and dip 
readings, shown on Plate XX. The general dip is southeasterly but 
locally the rocks dip steeply to the north. 

The iron-bearing belt has been traced by drilling toward the Rowe 
mine from the northeast through the north half of section 17. Where 
it enters section 18 at the east end of the Rowe pit, the iron-bearing belt 
is about 200 feet wide. Its width gradually increases to the southwest, 
and near the middle of the pit it has a width of 300 feet. This point 
marks the beginning of the synclinal turn to the north which occupies 
the entire west end of the pit. The synclinal fold is composed of a num- 
ber of minor folds which in turn are composed of smaller folds and 
crenulations. (Plate XVIII B). Around the end of the turn the out- 
lines of the iron-bearing belt are very irregular, and its width varies 
between 300 and 600 feet ; but beyond the turn on the north limb of the 
syncline the belt again becomes regular, and decreases in width to be- 
tween 200 and 300 feet. The turn of the syncline is very sharp, the north 
limb running back in a northeasterly direction practically parallel to the 
south limb. Both limbs have a general steep dip to the southeast. The 
bottom of the syncline pitches to the east between 45 and 6o°. 

The iron-bearing belt is bounded on both sides by schist and slate. 
South of the syncline and extending around the west end, is an extensive 
area underlain by gray and green chloritic and argillaceous schist and 



i#) GEOLOGY OF EAST CENTRAL MINNESOTA 

slate. Within the syncline, overlying the iron-bearing formation, are 
light gray siliceous sericitic slate and red ferruginous slate. The 
latter commonly occurs near the contact of the iron-bearing forma- 
tion, but is also found within the gray schist. The contact be- 
tween the red and gray slate is usually irregular and in most 
cases the red color is due to local staining. Slate or schist are found in 
the pit in only a few places, the principal exposures being along the center 
of the north side. The open pit, therefore, is almost entirely in ore and 
iron-bearing rock. 

The iron-bearing rocks at the Rowe mine consist of three distinct types 
of materials: (i) medium grade hematite and hydrated hematite ore 
(direct-shipping ore), (2) wash ore, consisting of hematite and hydrated 
hematite mixed with more or less disintegrated chert, and (3) hard fer- 
ruginous chert. These materials are somewhat irregularly distributed 
and frequently occur intermixed. In some parts of the pit, however, 
good ore predominates, and in other parts wash ore and ferruginous chert 
predominate. The latter especially seems to be concentrated along certain 
lines as is shown on Plate XX. The largest of these areas of ferruginous 
chert passes lengthwise through the center of the eastern portion of the 
pit, while another large area occurs in the southern part of the western 
half of the pit and trends northeasterly. Smaller masses are intermixed 
both with the wash ore and the direct-shipping ore. 

The direct-shipping ore differs from the wash ore and ferruginous 
chert in that it contains a smaller quantity of impurities such as chert 
or clay. It consists of a mixture of brown, red, and blue hematite, most 
of it more or less hydrated. The direct-shipping ore is the principal 
material in the pit. 

The ferruginous chert consists of intermixed hematite and chert. 
Usually the chert is dense and hard and is white, gray, or pink 
in color. It may be interlaminated with the hematite in thin 
irregular layers, or it may be intermixed with it in specks and blotches. 
This interlayering of light and dark bands is very characteristic of ferru- 
ginous chert. In the leaner ferruginous chert the dark bands, instead of 
being composed of hematite, consist chiefly of chert which has been 
stained by iron oxide. Usually in such cases the layering is less regular, 
however. 

The wash ore differs from the ferruginous chert in that the 
chert layers are disintegrated and consist mainly of very fine sand, in 
places slightly consolidated. This sand is removed by washing and the 
iron oxide interlayered with it becomes concentrated, the richness of the 
concentrate depending on the degree of disintegration which the chert 
has suffered and on the amount of impurity which the hematite layers 
contain. The hematite interlayered with the chert in wash ore or fer- 



GEOLOGY OF THE PRINCIPAL MINES 



141 



ruginous chert may be brown, red, or blue, depending on the amount of 
hydration which it has undergone. 

On the southern side of the iron ore body is a small mass of man- 
ganiferous ore, carrying about 43 per cent iron, 15 per cent manganese, 
and 11 per cent silica, and is apparently a replacement of ferruginous 
slate. The quantity of manganiferous ore, however, is so small that it 
is not shipped separately, but mixed with the non-manganiferous 
iron ore. 1 ** 

The depth to which the Rowe ore body extends is not known. One 
drill hole near the center of the pit encountered ore to a depth of 360 
feet below the original rock surface, while another one located in the 
angle of the syncline, passed into ore at a depth of 304 feet and con- 
tinued in merchantable ore for no feet. Many encountered ore at a 
depth of more than 200 feet below the base of the glacial drift. The 
lower limit of the ore body is probably irregular, the depth varying in 
different parts, depending upon the attitude and texture of the rock 
strata. The ore will probably be found to extend to its greatest depth in 
the western portion of the pit. 

The northern limb of the syncline at the Rowe mine consists mainly 
of ferruginous chert, and as far as known contains but little ore. It has 
not been thoroughly explored, however, and good ore bodies may be 
found along it. A minor fold occurs in this limb in the northeastern part 
of section 18, and beyond this the iron-bearing belt continues northeasterly 
through the northwest corner of section 17 into section 8. It may be con- 
tinuous with some of the northern ore belts in sections 9, 3, and 2, T. 
46N., R. 29W., in the central part of the north range. However, this 
connection has not yet been traced out. 

HILLCREST MINE 

The Hillcrest mine, operated by the Hill Mines Company, is located 
in the NE*4 of SE^ of section 9, T. 46N., R. 29W., just northeast of 
Blackhoof Lake and about one mile west of Ironton. It is approximately 
on the line of strike of the iron-bearing belt which forms the southern 
limb of the syncline at the Rowe mine. Iron-bearing rock has been traced 
almost continuously between the two mines, but it is not yet possible to 
state definitely whether the iron-bearing formations at the two mines are 
at the same stratigraphic horizon. However, the iron-bearing rock and 
associated ore of the Hillcrest mine are known to continue southwestward 
through the south half of section 9 and into the northwest quarter of 
section 16, thus leaving a stretch of only about three fourths of a mile 
of little known territory between this point and the northeast extension 
of the iron-bearing belt from the Rowe mine. 



"•Ferguson, H. G. 



142 GEOLOGY OF EAST CENTRAL MINNESOTA 

Hydraulic stripping operations at the Hillcrest mine were in progress 
during the summer and autumn of 191 5, and during a large part of the 
winter until stopped by excessive cold. During the summer of 191 6 the 
final stripping was being done with steam shovels. The overburden is 
65 to 70 feet thick. The upper part is sand and the lower part is clay 
containing locally boulders of considerable size. 

The iron-bearing rocks and associated ore are known mainly as a 
result of drilling. The drilling indicates that the iron-bearing belt has 
a maximum width of 400 feet and is mainly ore over most of its width. 
The strike of the iron-bearing belt is approximately N./O c E. and the dip 
is to the south at a fairly steep angle. Locally the beds are vertical. 
Gray sla'e and schist bound the iron-bearing belt on the north and south. 
Near the contact with the iron-bearing rock there is red ferruginous slate 
or paint rock. 

Recently there has been opened at the Hillcrest mine a small lenticular 
mass of manganiferous iron ore within the iron-ore body. It is bounded 
on both sides by iron ore with fairly sharp dividing lines. The lens is 
about 200 feet long by 20 wide, but is somewhat irregular in outline. It 
represents an impregnation of a part of the ore body by manganese oxide. 

Below are given average analyses of ore dried at 21 2° F. from the 
Hillcrest mine. 121 

Average Cargo Analyses of Ore Dried at 212 F. from the Hillcrest Mine 

Loss by 

Fe P Si0 3 Mn Al.O, CaO MgO S ignition 

% % % % % % % % % 

Hillcrest 58.000 .238 5.500 .230 2.680 .290 .030 .001 6.420 

Hillcrest man- 
ganiferous 43.811 .230 6.1 10 9.880 



PENNINGTON MINE 

The iron-bearing belt of the Hillcrest mine is directly continuous 
northeastward with the iron-bearing formation of the Pennington and 
Armour No. 1 mines. The ore bodies, however, are not connected. 

The Pennington mine, operated by the Pennington Mining Company 
of Tod-Stambaugh Company, is an open pit mine situated in the SWL 
of NE^4 of section 10, T. 46N., R. 29W. It is about half a mile from 
the Hillcrest mine, and the intervening area, which is known as 
the Feigh property, contains iron-bearing rock throughout its length : 
with it is associated a considerable quantity of ore. The Pen- 
nington pit is about 1,200 feet long, east and west, and has a maximum 
width of 680 feet from crest to crest. On the east it is directly contin- 
uous with the Armour No. 1 open pit which has a length of 750 feet 



m Lake Superior Iron Ore Association, op. cit., p. 9, 191 7. 



GEOLOGY OF THE PRINCIPAL MINES 



143 



and a width of 550 feet from crest to crest. The depth of the pit formed 
by these two mines is about no feet near the center and about 70 feet at 
the west end. The overburden of clay and sand above the ore and rock 
surface averages between 70 and 85 feet thick. The old erosion surface 
of the ore and iron-bearing rock shows a difference in elevation of about 
15 feet in different parts of the pit. Locally ore has been excavated to 
a depth of 35 to 40 feet beneath the original surface (summer, 1917). 
The width of ore and rock exposed along the bottom of the pit between 
the slopes of sand and clay varies from about 400 feet near the center to 
about 100 feet at the west end. Along the length of the combined pit, 
ore and iron-bearing rock are exposed for about 1,500 feet. 

The iron-bearing rock as exposed in the Pennington portion of the 
pit is very largely ore, some of it of very high grade. Along the north 
side, however, at the base of the sand slope, there are a few places where 
hard ferruginous chert is exposed. In the south half of the pit there is 
a lens of hematitic and chloritic schist varying in width from 50 feet to 
120 feet. (Plate XXI.) It is bounded on the north by ore. On the 
south it is bounded at some places by ore and at others by ferruginous 
chert. Locally, especially along its north side, the schist lens consists of 
dark red, soft, hematitic schist or paint rock, but for the most part it is 
made up of dark green or grayish green fine-grained chloritic schist con- 
taining both chlorite and sericite. In general appearance and structure 
as well as in its relation to the enclosing sedimentary rocks, the chloritic 
schist suggests a squeezed and altered subsilicic igneous rock. This 
is true not only of the chloritic schist in the Pennington and Armour 
No. 1 mines, but elsewhere in the Cuyuna district, practically wherever it 
occurs. Analyses and microscopic examinations of this rock will prob- ' 
ably aid in determining its true character. 

The best ore in the Pennington mine is a soft, dark red to reddish 
blue hematite. It is usually porous and breaks into a mixture of sand 
and small fragments when handled with the steam shovel. In general 
it is thinly laminated, and in many places shows crumpling. With in- 
creasing silica this ore grades into ordinary laminated ferruginous chert. 
The ferruginous chert as it occurs in the Pennington mine consists of 
thin layers of white, pink, or gray chert interlaminated with similar layers 
of more or less pure hematite, or hydrated hematite or with layers of 
chert stained dark red or blue by hematite. Thus there are all gradations 
from chert with certain layers stained by iron oxide to ore with only a 
very small amount of chert. The different layers or laminae vary in 
thickness from mere seams to an inch or more. Usually the layers 
themselves show fine lamination parallel to the bedding. Some ferru- 
ginous chert, while showing distinct lamination, does not have continuous 
layers of different colored materials. The coloration may be irregular, 



144 GEOLOGY OF EAST CENTRAL MINNESOTA 

following certain layers for a short distance and then cutting across the 
lamination, in general giving the rock a blotched appearance. This is 
especially common in the low-grade ferruginous chert which consists 
mainly of silica partly stained by iron oxide. Where the rock consists 
of distinct layers of chert and iron oxide, the layering is usually quite 
regular and very thin laminae may be followed for several feet. This 
is well shown locally along the north wall of the pit. In some of the 
ferruginous chert, the chert layers are partly disintegrated and the rock 
is soft and readily broken. Elsewhere, however, the rock is dense and 
extremely hard. 

The following average cargo analysis of ore from the Pennington 
mine is given by the Lake Superior Ore Association. 122 

Average Cargo Analysis of Ore Dried at 212 F. Shipped from the Pewmmgton 
Mine during the Season of 1916 

Loss by 

Fe P Si0 2 Mn Al.O, CaO MgO S ignition 

% % % % % % % % % 

Per cent 58.09 .192 8.26 .31 3.13 .30 .05 .021 4.55 

In places in the Pennington portion of the pit, drag folding is con- 
spicuously shown. (Plate XXI.) Three or four well-developed drag 
folds occur, whose axes trend obliquely across the pit in a direction 
slightly more north of east than the general strike of the bedding. These 
drag folds pitch 15 to 30 southwest, and their axial planes dip south- 
east, indicating that the iron-bearing rock of the Pennington-Armour 
No. I pit is on the south limb of an anticline. This structure is distinctly at 
variance with the structure as shown at the Rowe mine, where presum- 
ably the same iron-bearing layer forms the south limb of a syncline. 
This discrepancy may indicate that there is a break in the iron-bearing 
belt between the Hillcrest and Rowe mines. 

ARMOUR NO. I MINE 

The Armour No. 1 mine, operated by the Inland Steel Company, is 
adjacent to the Pennington mine on the northeast and occurs on the 
extension of the Pennington ore body. It was operated during 191 2 and 
1913 as an underground mine, but was idle during 1914. In the spring 
and summer of 191 5 the overburden was stripped and at present the 
Pennington and Armour mines are both in one big open pit, the Armour 
No. 1 mine operations being at the east end and the Pennington mine 
operations at the west end. The first ore was shipped from Armour 
No. 1 open pit during the fall of 191 5. 

The Armour No. 1 shaft, a circular concrete shaft, is located south 
of the ore body in the southern part of the SEJ4 of NE l 4 of section io, 



118 Lake Superior Iron Ore Association, op. cit.. p. 11, 



GEOLOGY OF THE PRINCIPAL MINES 



145 



T. 46N., R. 29W. It is 340 feet deep and drifts were run northward to 
the ore body at the 200 foot and 300 foot levels. The shaft penetrates 
65 feet of overburden, and below this it is mainly in green and gray 
chloritic schist similar to that occurring in places in the present open pit. 
Locally the schistosity of this rock is not very prominent and the rock 
is quite massive. About 200 feet north of the shaft on the bed 
rock surface is the contact between the chloritic schist and the southern- 
most layer of iron-bearing rock. Iron-bearing rock with associated iron 
ore and manganiferous iron ore and lenses of green schist and paint rock 
occupy the northern part of the Armour No. 1 40-acre tract. (Plate 
XXI.) The Pennington-Armour No. 1 ore body proper is about 500 feet 
north of the shaft on the bed rock surface. 

The general strike of the rock beds is approximately N.70°E., with 
local variations, and the dip is steeply to the south. The ore body has the 
same general strike as the rocks. It is cut into two parts longitudinally by a 
lens of green schist and paint rock that varies up to 100 feet in thick- 
ness. This lens pinches out to the west at the boundary of the Pennington 
property, and to the east it splits into several thin layers separated by ore 
layers, and gradually pinches out as it approaches the Armour No. 2 
property. Unaltered green schist forms the center of the lens, and the 
paint rock is found along the contact with the ore. The part of the ore 
body south of the schist lens is bounded on the south by another belt 
of paint rock and green chloritic schist, which is apparently continuous 
with the schist lens on the Pennington property. The part of the ore 
body lying north of the northern schist lens grades northward into man- 
ganiferous iron-bearing rock, some of which carries 14 per cent or more 
of manganese. The ore body including the lens of paint rock and green . 
schist varies up to more than 200 feet in width and extends north- 
eastward across the entire 40-acre tract from the Pennington property 
line to the Armour No. 2 property line. 

The ore is of good grade, ranging on the average between 55 per cent 
and 62 per cent in metallic iron. It is of non-Bessemer quality, how- 
ever, ranging in general between 0.15 and 0.3 per cent phosphorus. Its 
appearance and texture are very similar to the ore on the Pennington 
property, and seem to be more or less uniform throughout the ore body. 
The following average cargo analysis of ore from the Armour No. 1 
mine is given by the Lake Superior Iron Ore Association. 123 

Average Cargo Analysis of Ore Dried at 212 F. Shipped from the Armour No. 1 
Mine during the Season 1916 

Fe P SiO, Mn Al 2 O a 

Per cent 58.27 .187 7.39 .29 2.47 



M Lake Superior Iron Ore Association, op. cit., p. 8. 



146 



GEOLOGY OF EAST CENTRAL MINNESOTA 



IRONTON MINE 

The I ronton mine of the Cuyuna-Duluth Iron Company is located 
just north of the village of Ironton, in the NW}4 of SYVJ4 of section 
ii, T. 46N., R. 29W. It consists of a lath shaft which passes through 
65 feet of glacial till into underlying green chloritic schist to a total depth 
of 300 feet. At the 200 and 280 foot levels, drifts run northwestward 
from it to the ore body, the distance being about 200 feet on the 200 foot 
level. (Plate XXL) The ore body is the southwesterly extension of 
the main Armour No. 2 ore body. It is more or less parallel to the 
Pennington-Armour No. 1 ore body and is about 1,000 to 1,200 feet 
southeast of it. Between the two ore belts are green chloritic schist, and 
paint rock with some lenses of lean iron-bearing rock. 

The ore of the Ironton mine is for the most part soft and granular 
hematite, dark red to reddish blue. It is of high grade and is similar 
to much of the ore in the western part of the Armour No. 2 mine. 

The following average cargo analysis of ore from the Ironton mine 
is given by the Lake Superior Iron Ore Association. 124 

Average Cargo Analysis of Ore Dried at 212 F. Shipped from Ironton Mine 

During the Season 1916 

Fe P SiO, Mn A1,0, 

Per cent 5970 .172 575 -8i 4-44 

ARMOUR NO. 2 MINE 

The Armour No. 2 mine of the Inland Steel Company, is located in 
the SW*4 of NW T /i of section 11, T. 46N., R. 29W. It consists of a 
circular concrete shaft 298 feet deep and a considerable amount of under- 
ground workings. The shaft penetrated 63 feet of glacial overburden, 
below which it encountered green chloritic schist. The main hauling level 
is at a depth of 168 feet. At this level, cross-cuts run north-northwest- 
ward and south-southeastward from the shaft. (Plate XXL) The 
first of these connects with the extension of the 200 foot level of the 
Armour No. 1 mine in the Pennington-Armour No. 1 ore body, and the 
second connects with the main Armour No. 2 ore body which continues 
northeastward from the Ironton mine through the Armour No. 2 
40-acre tract. Sublevels are found 127 feet and 107 feet below the sur- 
face, and a new exploration drift runs south-southeastward from the 
shaft at the 258 foot level. 

The general strike of the main Armour No. 2 ore body is about N. 
55°E. The Pennington-Armour No. 1 ore body strikes N.70°E., thus 
indicating a convergence of the two ore bodies to the northeast. The 
main ore belt has a maximum width of more than 400 feet, and, as the 



m Lake Superior Iron Ore Association, op. cit., p. 9. 



GEOLOGY OF THE PRINCIPAL MINES 



147 



dip is at a fairly high angle to the southeast, this gives considerable thick- 
ness to the ore body. There is, however, some lean ferruginous chert 
mixed with the ore, as is the case in nearly all the ore bodies in the 
Cuyuna district. 

The ore body is of considerable longitudinal extent, for the indica- 
tions are that it continues northeastward through the adjoining 
40-acre tract which also forms part of the Armour No. 2 property, and 
through the Thompson, Meacham, and Croft properties, a total length 
of more than one and three fourths miles. However, not enough drilling 
has been done as yet definitely to prove the continuity of the ore. 

There is a curious distribution of different kinds of ore in 
the ore body on the Armour No. 2 property. The southwestern 
part of the body, to a point a short distance northeast of the 
main cross-cut, consists of dark red and reddish blue hematite, 
usually soft and granular and but slightly hydrated. This ore 
is high in metallic iron, ranging up to 66 and 67 per cent, and is com- 
paratively low in phosphorus, much of it varying between .02 and .07 
per cent. Probably a considerable amount of ore of Bessemer grade 
could be mined by careful selection. The northeastern part of the ore 
body consists of an entirely different type of ore. It is largely a medium- 
hard, brown ore consisting of limonite and hydrated hematite. This ore 
continues northeastward into the eastern Armour No. 2 40-acre tract, 
where it locally becomes highly manganiferous, although it contains less 
manganese than the foot-wall of the Armour No. 1 ore body. The man- 
ganiferous iron ore appears to occur as local lenses interlayered with ore 
and ferruginous chert which is practically free from manganese. 

Southeast of the main Armour No. 2 ore body, and forming the 
hanging wall throughout most of its extent, are beds of hard and rather 
lean ferruginous chert. Locally, however, in the southwestern half of 
the ore body, some of the drifts have encountered red, ferruginous slate 
in the hanging wall directly in contact with the ore. It is probable that 
drifts driven southward through the ferruginous chert will penetrate it 
and encounter red hematitic and green chloritic schist south of it similar 
to that which occurs in the hanging wall of the ore body at the Ironton 
and Thompson mines. 

The foot-wall of the main Armour No. 2 ore body is formed by a 
layer of paint rock or hematitic schist, which, however, is only 10 or 15 
feet thick, and grades into compact green chloritic schist. (Plate XXI.) 
The latter probably continues northward to the Armour No. 1 ore 
body, containing locally, however, a few lenses of iron-bearing formation. 
The main cross-cut running north-northwestward from the shaft to the 
extension of the Armour No. 1 ore body is entirely in chloritic schist. 



148 



GEOLOGY OF EAST CENTRAL MINNESOTA 



The presence of local layers or lenses of iron-bearing formation, how- 
ever, such as occur also in the Armour No. I property, has been shown 
by drilling. 

The extension of the Pennington-Armour No. i ore body in the 
Armour No. 2 property is comparatively thin and appears to become 
thinner to the northeast. Compared to the main ore body, it is of no 
great importance. Its hanging wall consists of green chloritr schist, 
while the foot-wall is formed by iron-bearing formation which is locally 
manganiferous like that in the Armour No. I property. The ore is simi- 
lar to that in the Armour No. I and Pennington mines. 

Three grades of ore are shipped from the Armour No. 2 mine. 1 " 

Average Cargo Analyses of Ore Dried at 212° F. Shipped from the Artmonr No. 2 
Mine during the Season 1916 



Fe 

Red (per cent) 58.20 

Brown (per cent) 54-8o 

Manganese (per cent) 49-30 



p 


SiO, 


Mn 


AI.O. 


.104 


9.05 


•19 


300 


.281 


6.94 


1. 05 


3-92 


•215 


8.00 


528 


3-43 



THOMPSON MINE 

The Thompson mine of the Inland Steel Company consists of both 
open pit and underground workings. The latter, however, were not used, 
except for drainage purposes, between 1913 and 1916, after which date 
steam shovel operations gave place to milling. The shaft is located in the 
southern part of the NW*4 of NE*4 of section 11, T. 46N., R. 29W., and 
the open pit is just south of it on the line between this 40-acre tract and 
the one immediately to the south. 

The shaft, which is 305 feet deep, has cross-cuts north and south at 
the 150 and 250 foot levels. There is also a sublevel at 100 feet not 
connected with the shaft. The north cross-cuts encounter an important, 
but as yet little developed, ore lens located in the northern part of the 
40-acre tract in which the shaft is located, and in the southern part 
of the 40-acre tract adjoining it on the north. This lens is on the line 
of strike of the Pennington-Armour No. 1 ore body, but drilling has 
shown that it is a separate lens although it occurs at approximately the 
same horizon. The south cross-cuts connect with an ore body imme- 
diately south of the shaft which is the northeastward extension of the 
main Armour No. 2 ore bodv, as has already been mentioned. (Plate 
XXII.) 

The Thompson open pit, which is on the south ore body, has a length 
of about 1,600 feet and a maximum width from crest to crest near the 
middle of 450 feet. The overburden, consisting of sand above and of 
gray and red clay and sand below, varies in thickness from 50 feet to 



ta Lake Superior Iron Ore Association, op. cit., p. 8. 



GEOLOGY OF THE PRINCIPAL MINES 



149 



80 feet depending upon the topography. The ore surface is compara- 
tively level, although it rises somewhat toward the west. Ore and fer- 
ruginous chert are exposed along the bottom of the pit practically through- 
out its entire length. The width of the bottom of the pit is approximately 
100 feet. The bottom in the west end of the pit in the fall of 191 5, was 
about 50 feet below the original ore surface, and in the fall of 19 16 about 
90 feet below. The strike of the beds is very regular throughout the 
length of the pit and is about N.65°E. The dip is to the southeast and 
varies between 50 and 75 °, averaging about 65 °. 

The foot-wall of the ore body which forms the north wall of the 
lower part of the pit, consists of light gray to dark gray or red siliceous 
and ferruginous argillite (or slate). It is very thin-bedded and, due to 
the abundance of silica, has only locally developed slaty cleavage. In 
places it is stained dark red by hematite. Ferruginous layers are 
interbedded with siliceous and argillaceous layers. Just below the main 
ore body a thin lens of ore is interbedded with the foot- wall argillite. 

The ore of the main ore body occupies most of the bottom of the pit. 
In places it consists of almost pure dark brown limonite, while elsewhere 
it is mixed with sandy disintegrated ferruginous chert or with hard, 
dense, ferruginous chert. A thin layer of red, ferruginous argillite oc- 
curs interbedded with ore in the western part of the pit. The ore as a 
rule is richest near the foot-wall and for 50 or 60 feet away from it. 
Toward the hanging wall it becomes more siliceous. Locally along the 
foot-wall between the ore and the argillite are irregular lenses of man- 
ganiferous ore. 

The south wall of the pit consists in the western part of hard, massive, 
ferruginous chert, and in the eastern part of soft, dark red hematitic 
schist, which is doubtless decomposed and iron-stained chloride schist, 
and forms the hanging wall of the iron-bearing formation. 

Most of the ore is shipped directly, as it is loaded by steam shovels in 
the pit, but the hard ferruginous chert occurring with it irregularly along 
certain horizons is not used. The soft, more or less disintegrated, fer- 
ruginous chert is concentrated in a small washing plant equipped with log 
washers. The following average cargo analysis of ore from the Thomp- 
son mine is given by the Lake Superior Iron Ore Association. 126 

Average Cargo Analysis of Ore Dried at 212 F. Shipped from the Thompson 
Mine during the Season 1916 

Fe P Si0 2 Mn A1,0, 

Per cent 52.34 .250 8.30 2.38 3.44 



■•Lake Superior Iron Ore Association, op. cit., p. n. 



ISO GEOLOGY OF EAST CENTRAL MINNESOTA 

The north ore lens of the Thompson mine, as indicated by the drill- 
ing, is considerably larger than the south lens. It has a length of about 
2,000 feet and a maximum width of 600 feet. Much of it is manganifer- 
ous, and it contains considerable iron-bearing rock mixed with the ore. 
Like the south ore lens, it dips steeply to the southeast. Between the 
north and south lenses is a great thickness of slate and green chloritic 
schist with bands of iron-bearing formation. 

MEACHAM MINE 

The Meacham mine, operated by Rogers-Brown Ore Company, is 
situated just east of the Thompson open pit on the continuation of the 
upper or southern Thompson ore body. The property is an 80-acre tract 
including the NE}4 of NE*4 of section 11, and the NW*4 of NW*4 of 
section 12, T. 46N., R. 29W. 

The shaft, which is located in the of NE}4 of section 11, is a 

circular concrete shaft having a total depth of 254 feet, of which the 
upper 70 feet are in glacial drift. The main hoisting level is at 154 feet, 
while the principal working levels at present are at 85, 105, and 115 feet. 
The shaft is in the foot-wall of the ore body which, like the Thompson 
ore body, dips steeply to the southeast. The association of ore and wall 
rocks is similar in the two mines. 

The following average cargo analyses of ore from the Meacham mine 
are given by the Lake Superior Iron Ore Association. 127 

Average Cargo Analyses of Ore Dried at 212° F. Shipped from the Meacliam 
Mine during the Season 1916 

Loss by 





Fe 


P 


SiO, 


Mn 


A1,0. 


CaO 


MgO 


S ignition 




% 


% 


% 


% 


% 


% 


% 


% % 


Meacham 


















Bessemer. . 


59.90 


•045 


3-50 


40 










Meacham Non 


















Bessemer. . 


55-00 


•3347 


5889 


•535 


3.216 


.536 


.272 


0139 10.547 


Meacham 


















Manganese 


46.58 


.308 


6.52 


4.42 


4.22 


2.08 


•55 


12.02 








CROFT MINE 











The Croft mine, operated by John A. Savage and Company, is situated 
one half mile northeast of the Meacham mine on the same iron-bearing 
belt as the Armour No. 2, Thompson, and Meacham mines. The prop- 
erty comprises the SE}4 of SWJ4 and the SEy 2 of S\W l / 4 of SWJ4 of 
section 1, T. 46N., R. 29W. The western portion of it adjoins the eastern 
Meacham 40-acre tract on the north. 



177 Lake Superior Iron Ore Association, Analyses of Lake Superior iron ores, season 1916, 
pp. 10 and 11, 1917. 



GEOLOGY OF THE PRINCIPAL MINES 



151 



A circular concrete shaft located a short distance northwest of the 
iron-bearing belt, was sunk through no feet of surface drift to the ledge. 
From its bottom a rectangular steel shaft passes through rock forming 
the foot-wall of the ore body to a total depth of 250 feet. The main 
cross-cut, on the 222-foot level, connects the shaft with the various drifts 
and cross-cuts in the ore body. The main working level is at 160 feet. 

Two bands of iron-bearing rock cross the Croft property in an ap- 
proximately northeast-southwest direction. They are separated by a 
wedge of green chloritic schist and associated red hematitic schist, which 
widens to the northeast and comes to a point in the southwestern part 
of the property. On the northwest and southeast, the iron-bearing belts 
are bounded by green and red schists. Those to the southeast are chlo- 
ritic and those to the northwest are locally quartzose, and are probably 
sedimentary in origin as indicated by their banded and laminated char- 
acter. The dip of the iron-bearing beds averages about 6o° southeast. 

The upper iron-bearing bed, i.e., the one to the southeast, consists 
mainly of ferruginous chert and slate associated with low grade ore, some 
of which is manganiferous, and with lenses of green and red schist. It 
has a maximum width of 350 feet on the erosion surface. The mine is 
located on the lower iron-bearing bed. This bed has an average width of 
180 to 200 feet on the erosion surface, but is rather irregular. A part of 
the layer consists of ferruginous chert associated with some ferruginous 
slate, and another part of it consists of rich, dark reddish blue hematite 
which occurs as a layer of varying width running practically through the 
entire length of the property. Most of the ore, according to drill records, 
is of Bessemer quality and is the richest ore that has been found in the 
Cuyuna district. The following average analysis of ore from the Croft 
mine is given by the Lake Superior Iron Ore Association. 128 

Average Cargo Analysis of Ore, Dried at 212° F., Shipped from the Croft 
Mine during the Season igi6 

Loss by 

Fe P Si0 2 Mn A1 2 3 CaO MgO S ignition 

Per cent 597i8 .0435 10.250 .110 1.630 .290 .030 .0010 2.600 

The depth to which the Croft ore body extends has not yet been 
shown by drilling. Throughout most of the property it dips southeast 
more or less parallel to the bedding of the associated rocks and retains a 
fairly regular thickness with depth. The indications that the ore may 
extend to a considerable depth are probably as favorable here as they 
are in any other deposit in the district. 



158 Lake Superior Iron Ore Association, op. cit., p. 8. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



CUYUNA-MILLE LACS MINE 

The Cuyuna-Mille Lacs mine of the Cuyuna-Mille Lacs Iron Com- 
pany, is located in the SE}4 of SWJ4 of section 3, T. 46N., R. 29W., 
southeast of Menomin Lake, but the property includes also the NWJ4 
and SVJ% of SW*4 of section 3. The main hoisting shaft, a lath shaft, 
was sunk through about 50 feet of glacial till and 175 feet of ore and 
rock to a depth of 225 feet. The timber shaft is 205 feet deep and the 
main hauling level is at the bottom of it. Both shafts are sunk directly 
to the Cuyuna-Mille Lacs north ore body. The principal mining opera- 
tions are now on the 85, 125, 145, 165, and 205 foot levels. At the 
present time (summer 191 7) a second hoisting shaft is being sunk in the 
SWJ4 of SW34 of section 3. This will connect directly with the south 
ore body. 

The Cuyuna-Mille Lacs north ore body occurs along the crest of an 
anticline which strikes approximately northeast-southwest. The north 
limb of the anticline near the ore body has a steep dip northwest, but away 
from the ore body the beds are overturned, and have a steep dip to 
the southeast. The south limb of the anticline dips southeast. The dip 
is fairly low near the ore body, but is higher toward the southeast. The 
ore body appears to occupy a somewhat indefinite stratigraphic horizon, 
but its position along the crest of the anticline and its continuation down- 
ward along the axial plane suggests that in part, at least, the ore body- 
is a replacement of a fractured zone. Ferruginous chert bounds the ore 
body on the northwest and southeast. The most characteristic wall rock 
is an evenly laminated, medium hard, ferruginous chert, which cleaves 
readily along lamination planes, breaking in thin slabs. This rock is 
found on both sides of the ore body, but is more characteristically de- 
veloped on the south side. Some phases of it are rather argillaceous, 
approaching ferruginous slate in character. At some points along the 
south side of the ore body, there is a layer of hard, jaspery, ferruginous 
chert. It is bounded on the north side by low-grade, soft, manganiferous 
iron ore, and on the south side by the laminated, argillaceous ferruginous 
chert already mentioned. Up to the present time this jasper layer has 
not been encountered on the north limb of the anticline. It is probably 
a local lens. 

The structure of the Cuyuna-Mille Lacs anticline with its north limb 
dipping steeply northward or overturned, and its south limb dipping 
steeply southward, suggests that it may be a drag fold on the south limb 
of a more extensive major anticline. South of the ore body for some 
distance, southeasterly dips of varying steepness prevail, the rocks being 
mainly iron-bearing formation or manganiferous iron-bearing forma- 
tion. In the southern part of the SEJ4 of SWJ4 of section 3, however, 



GEOLOGY OF THE PRINCIPAL MINES 



153 



a southwestward pitching syncline occurs which is apparently continu- 
ous to the northeast with the synclinal trough in the northern part of 
the Sultana property. The rock strata, however, composing the syncline 
differ from those found in the Sultana syncline in being more cherty, 
though slaty layers occur here and there. Several extensive ore lenses 
are found in the syncline and adjacent to it. They are known collectively 
as the south ore body. 

South of the Cuyuna-Mille Lacs property, in the northern part of 
section 10 is the Mahnomen open pit. Steep southeasterly dips prevail 
in the ore at this place, and no secondary structures which might aid in 
determining the relation of this ore belt to the major structure have so 
far been found. Lithologically, however, certain manganiferous iron 
ore layers found in the southern part of the Mahnomen pit are very 
similar to the manganiferous iron ore of the Sultana and Armour No. 
i mines* and they may represent the same stratigraphic horizon. As 
already mentioned, the drag folds found along the Pennington-Armour 
No. i ore belt south of the Mahnomen ore belt, indicate that the beds 
here are on the south limb of a major anticline. Not enough data are 
as yet available to determine the structural relations of these various 
beds. It seems probable, however, that a series of folds exists between 
Menomin Lake and the Pennington pit and results in a repetition of 
layers. Between which of the ore beds there are anticlines and between 
which there are synclines can not yet be stated definitely. 

The iron-bearing belt in which the Cuyuna-Mille Lacs ore body oc- 
curs has been traced continuously only from the southern part of sec- 
tion 4, T. 46N., R. 29W., northeastward through section 3. Little is, 
therefore, known of its longitudinal extent. 

The Cuyuna-Mille Lacs north ore body is compact in form, but the 
south ore body consists of several separate masses. Both vary con- . 
siderably in the character of the material from place to place. Locally 
bodies of rich black manganese ore occur, but the general run of material 
is a medium soft to hard black or brownish manganiferous iron ore which 
varies in manganese content from place to place. The rich manganese 
ore as a rule is finely crystalline pyrolusite; some is fairly hard and 
compact and some is soft and friable. Dense, hard, black psilomelane 
is also of frequent occurrence. The manganiferous iron ore usually has 
a blocky structure and is dense and amorphous. Specks of crystalline 
material, however, frequently occur scattered through it. 

Four grades of ore are mined by the Cuyuna-Mille Lacs Iron Com- 
pany ; the average cargo analyses are as follows : 129 



128 Lake Superior Iron Ore Association, op. ext., p. g, 1917. 



154 GEOLOGY OF EAST CENTRAL MINNESOTA 

Average Cargo Analyses of Ore Dried at 21 2° F. Shipped from Cuyuna-Mille Lacs 

Mine, for the Season 1916 









Fc 


P 


SiO, 


Mn 


A 1,0, 


CaO 


MgO 








% 


% 


% 


% 


% 


% 


% 


Crow 


Wing 


" A", 


■ • 3771 


.091 


11.46 


21.22 


•95 


.82 


47 


Crow 


Wing 


V 


• • 3902 


.096 


1320 


17.06 


42 


&4 


.67 


Crow 


Wing 


"C" 


• • 37-39 


79 


19.50 


12.91 


.220 


.670 


63 


Crow 


Wing 


"D" 




.051 


22.00 


8.96 


.182 


.68 


.60 



The Cuyuna-Mille Lacs mine has been by far the largest shipper of 
manganiferous iron ore in the Cuyuna district. When the mine was 
first opened, considerable high-grade ore was produced, but more recently 
the reserves have been increased by reason of the fact that lower grade 
ore in large quantities is being shipped. 130 The Cuyuna-Mille Lacs ore 
contains a considerably lower percentage of phosphorus than the general 
run of iron ore of the Cuyuna district. 

MANGAN NO. I MINE 

The Mangan No. 1 mine of the Mangan Iron and Steel Company is 
situated in the NE^ of SWJ4 of section 3, T. 46N., R. 29W., just north- 
east of the Cuyuna-Mille Lacs mine, being on the northeastern extension 
of the same ore body. The shaft, a round timber shaft, was sunk to a 
depth of 122 feet, of which 60 feet are in surface drift and 62 feet in 
slate and slaty iron carbonate rock. The ore body is south of the shaft. 
Some ore was mined in the summer and autumn of 1916 and mining 
continued during 191 7. 

SULTANA MINE 

The Sultana mine, situated east of the Cuyuna-Mille Lacs mine in 
the SWJ4 of SE>4 of section 3, T. 46N., R. 29W., is operated by the 
Sultana Mines Company. It is in the southern part of the same belt of 
iron-bearing formation on which are located the Cuyuna-Mille Lacs and 
Mangan No. 1 mines and the Hopkins shaft. As already mentioned this 
belt of iron-bearing formation consists typically of a finely laminated, 
somewhat argillaceous ferruginous chert in the vicinity of the Cuyuna- 
Mille Lacs ore body. To the southeast across the strike this grades into a 
denser, more siliceous, ferruginous chert with lamination planes less 
marked. The portion of the belt along the northeastward extension of 
which the Sultana mine is located is still farther southeast and consists 
predominantly of ferruginous slate with local layers of hard, siliceous, 
ferruginous chert or of more or less decomposed ferruginous chert. 



"Zapffe, Carl, Matters of interest to operators regarding the Cuyuna district: Proc. Lake 
Superior Mil*. Inst., vol. 20, p. 197, 1015. 



GEOLOGY OF THE PRINCIPAL MINES 



155 



The ore body at the Sultana mine is located in the trough of a north- 
east-southwest trending syncline 131 with a gently southeastward dipping 
north limb and a steeply northwestward dipping south limb. It appears 
to occupy a position farther south relative to the general trend of the 
iron-bearing belt than the Cuyuna-Mille Lacs anticline, but is more or 
less parallel to it. This relation seems to indicate that the Cuyuna-Mille 
Lacs-Sultana iron-bearing belt contains a series of more or less parallel 
folds whose axial planes dip uniformly to the southeast. The degree of 
folding varies in different parts, but in general it has been intense, so 
that isoclinal folds have resulted and the general dip of the beds also is 
to the southeast. Locally, however, near the axes of the folds north- 
westerly dips occur. 

With the exception of a few small bands of schist, the Sultana 40- 
acre tract is underlain by iron-bearing rock and associated ore. The 
bands of schist have a northeast-southwest trend, parallel to the strike 
of the iron-bearing beds. One band occurs near the southeast corner of 
the property, north of the eastern end of the Mahnomen open pit, and 
the other is in the northeastern part. In the northern part of the prop- 
erty, where the ore body occurs, the iron-bearing rock consists mainly 
of laminated ferruginous slate of which there are two distinct varieties. 
One of them is the common dark red, medium hard, hematitic slate (or 
hematitic argillite) and the other a brownish or yellowish, soft, plastic, 
limonitic clay which becomes medium hard and compact on drying. 

As far as can be seen in the present underground workings, the soft, 
limonitic clay occurs along the northern line of the 40-acre tract and 
continues some distance southward. It is everywhere finely laminated, 
due to interlayered light and dark colored laminae. The general strike 
of the bedding is somewhat north of east. The dip in parts is low to the 
southeast and in other parts steeply to the northwest. The dark red • 
hematitic slate has been encountered only in a few places. It lies south 
of the soft limonitic clay and dips steeply to the north or northwest. 
Ore occurs in both varieties of ferruginous slate, but most of it is in the 
limonitic phase of the slate. 

The main hoisting shaft was sunk directly in the portion of the ore 
body found in the limonitic clay about 200 feet south of the north line 
of the property. It is 91 feet deep, and at the bottom of it is the main 
hauling level. Other levels occur at depths of 65 feet and 75 feet. 
Cross-cuts run from the shaft northward through the ore body and 
southward into the dark red hematitic slate. A drift running westward 



m Much of the information concerning the Sultana mine was kindly furnished the 
writers by Mr. H. H. Bradt, Mines Efficiency Co., Duluth, Minn., and Mr. E. Newton, School 
of Mines Experiment Station, Minneapolis, Minn. 



j 56 GEOLOGY OF EAST CENTRAL MINNESOTA 

from the main shaft to an exploration shaft in the northwest corner of 
the property passes through much manganiferous ferruginous slate and 
chert. Ferruginous chert also occurs locally interlayered with the limo- 
nitic clay. 

The manganiferous iron ore at the Sultana mine consists of: (i) 
irregular nodules, lumps, and masses of different sizes scattered through 
the yellow and brown limonitic clay, and (2) veins and nodules in the 
dark red hematitic slate (or hematitic argillite). The ore in the limo- 
nitic clay is usually dense, amorphous, and hard, although considerably 
broken up. Its distribution in the clay is most irregular, appearing to be 
in the form of local replacements. 131 ' The limonitic clay matrix carries 
only a very small percentage of manganese except locally, where masses 
of it appear to have been partly replaced by manganese oxide, but not 
so completely as to convert it into ore. The origin of the limonitic clay 
is not clear. It may have been derived from the dark red hematitic slate 
by hydration and leaching, or the limonitic clay and hematitic slate may 
both be decomposed phases of ferruginous argillaceous rock which differed 
somewhat in character and were interbedded with each other. The ore in 
the hematitic slate is usually very hard and characteristically occurs along 
joints and fissures. Locally elongated nodules of manganiferous iron 
ore are found along joints where solutions following the joints have 
had an opportunity to penetrate the wall rocks on either side. Else- 
where thin veins of nearly pure manganese oxide occur along fissures. 
Such veins are bounded on both sides by bands of low-grade, reddish 
black, manganiferous iron ore representing a partial replacement of wall 
rock, and these low grade bands in turn are bordered with sharp con- 
tacts by unaltered hematitic slate. 

The following is an average cargo analysis of ore from the Sultana 
mine : 133 

Average Cargo Analysis of Ore, Dried at 212 F., Shipped from the Sultana 
Mine during the Season 10 16 

Loss by 

Fe P SiO, Mn A1,0, CaO MgO S ignition 

Per cent 39-302 .170 8.536 13.426 3-50 75 -50 .015 10.00 

HOPKINS MINE 

The Hopkins property includes the N*/2 of SE}4 of section 3, T. 46N., 
R. 29W., the shaft being located in the of SE*4 of section 3, just 



"^The ore body at the Sultana mine is a very interesting one as regards itructure and 
origin. A detailed examination of the mine was made by Mr. Harlan H. Bradt, of Duluth, and 
Mr. Edmund Newton, of Minneapolis, who suggested to the writers the probability that the man- 
ganiferous iron ore of the Sultana mine originated by the replacement of an original iron-bearing 
rock or manganiferous iron-bearing rock by manganese oxide. Subsequently one of the writers 
(Harder) made an examination of this and other Cuyuna Range manganiferous iron-are deposits 
and found that the replacement hypothesis was probably applicable to all of them without ex- 
ception. 

■ Lake Superior Iron Ore Association, op. tit., p. It. 



GEOLOGY OF THE PRINCIPAL MINES 



north of the Sultana mine. The shaft has a depth of 145 feet, about 50 
feet being in glacial drift and the rest mainly in a hard, jaspery, ferru- 
ginous chert and jaspilite. No ore has been shipped from the mine, the 
shaft having not yet (summer, 191 7) reached the ore body. 

The Hopkins mine is located along the same general stratigraphic 
horizon on which the Cuyuna-Mille Lacs and Mangan No. I mines are 
located, and the jaspilite and hard ferruginous chert which occur in the 
shaft resemble similar rocks found in the Cuyuna-Mille Lacs mine. 
Some layers are dark, reddish black, and massive and some are finely 
laminated and black, dark red, or brown. Laminae of amphibole or 
hematite, and locally also of carbonate minerals, occur interlayered with 
the laminae of dark colored chert. Many jasper layers have a well- 
developed oolitic texture. The rock in general is hard and siliceous. 

The shaft was sunk for the purpose of opening up a body of rich 
hematite ore, but operations were discontinued before the ore was 
reached. The southern part of the Hopkins property contains the north- 
eastward continuation of the Sultana iron-ore belt. 

JOAN MINE"* 

Thia is a new mine, operated by the Joan Mining Company, 
located in the SWJ4 of NE*4 of section 3, T. 46N., R. 29W., from 
which shipment of manganiferous ore has just begun. The ore con- 
sists of a replacement by manganese oxides of cherty ferruginous ma- 
terial. The outlines of the ore body are irregular, and development work 
has not yet been carried far enough to determine the extent of the ore. 
It appears to be a band about 12 feet wide following a generally northerly 
direction. 

MAHNOMEN MINE 

The Mahnomen open-pit mine of the Mahnomen Mining Company 
is in the northern part of section 10, T. 46N., R. 29W. Most of the 
pit is in the NE}4 of NW}4 and the NWJ4 of NEJ4 of the section, but 
the eastern end of it projects for a short distance into the 40-acre tract 
of the Sultana Mines Company, as well as into the 40-acre tract of the 
Mangan Iron and Steel Company which comprises the NE}4 of NEJ4 
of section 10. The location is about half-way between the Pennington 
open pit and the Sultana mine. 

The Mahnomen ore body occurs along the northern edge of a broad 
belt of iron-bearing rock which extends in a southwest-northeast direc- 
tion through the northern part of sections 10 and 11, the southeastern 
part of section 3, and the southern part of section 12, T. 46N., R. 29W. 
To the southwest it can be traced with interruptions through section 9, 



•* Fd*aeon, H. G. 



158 



GEOLOGY OF EAST CENTRAL MINNESOTA 



T. 46N., R. 29W., into section 8, while to the northeast it continues 
through the northern part of section 1, T. 46N., R. 29W., and through 
section 31, T. 47N., R. 28W., toward the southeastern part of Rabbit 
Lake. The details of the structure of this iron-bearing belt, and its 
relation to the Pennington-Armour No. 1 belt on the south and the 
Cuyuna-Mille Lacs-Sultana belt on the north, have not been determined. 
The belts appear to merge into each other, although locally they are sep- 
arated by schist bands. Development work is constantly progressing, and 
it will doubtless soon be possible to determine some definite relation be- 
tween them. 

The Mahnomen open pit contains both iron ore and manganiferous 
iron ore. The iron ore is mainly of the yellow and brown ocherous 
variety, of porous texture and finely laminated structure, which is not 
very common on the north range, but is abundant on the south range. 
Layers of soft red ore, however, are interlayered with the yellow ocher- 
ous ore. Little or none of the hard cherty ore common in the Thompson 
and Armour mines occurs in the Mahnomen open pit. The lamination 
of the ore is parallel to the general strike of the iron-bearing belt, being 
somewhat north of east. It dips steeply south for the most part, but 
locally is vertical, and in many places crumpling has resulted in northerly 
dips. 

The iron ore of the Mahnomen mine is found along the north side 
of the pit. To the south, across the strike of the beds, it grades into 
dark red ferruginous slate, locally impregnated with manganiferous iron 
ore, which in general occupies a band running lengthwise through the 
center of the pit. The south side of the pit is in manganiferous iron 
ore associated with yellow clay and some red ferruginous slate. The 
ore generally occurs as veins and as large and small nodular or irregular 
masses imbedded in soft yellowish and brownish clayey and slaty ma- 
terial. Some ore is also found in the ferruginous slate layers interbedded 
with the yellowish and brownish clay, but is less abundant and less rich. 
The ferruginous slate layers are not numerous along the south wall of the 
pit, but increase in abundance northward on approaching the red slaty 
horizon separating the manganiferous iron ore from the iron ore. Here 
and there are thin layers of gray or variegated, finely laminated, plastic 
clay very similar to material occurring in the Sultana mine. As has 
been mentioned, the entire manganiferous bed is strikingly like that of 
the Sultana mine and may represent the same stratigraphic horizon. 

The following grades of ore are offered by the Mahnomen Mining 
Company, according to the year-book of the Lake Superior Iron Ore 
Association. 131 



m Lake Superior Iron Ore Association, op. cii., p. 10. 



GEOLOGY OF THE PRINCIPAL MINES 



159 



Averagt Cargo Analyses of Ore Dried at 212 F. Offered by Mahnomen Mining 

Company for the Season 1917 

Loss by 



Fe 




P 


SiO 


Mn 


Al 2 O s 


CaO 


MgO 


S 


ignition 




% 


% 


% 


% 


% 


% 


% 


% 


% 


Mahnomen 


57.50 


.19 


6.10 


.80 


2.67 


.98 


.42 


•054 


7.84 


Mahnomen No. 1 


40.00 


•15 


10.00 


14.00 


3.80 


.22 


•31 


.015 


7.84 


Mahnomen No. 3 


46.00 


.20 


7-55 


8.00 


3.80 


.22 


•31 


.015 


9.84 


Mahnomen No. 4 


5100 


•19 


6.10 


450 


3-70 


75 


1.24 


.029 


9.00 



MANGAN NO. 2 MINE 

The Mangan No. 2 mine of the Mangan Iron and Steel Company is 
located in the NE}4 of NE}4 of section 10, T. 46N., R. 29W., just north 
of the Armour No. 1 40-acre tract and east of the Mahnomen open 
pit. The southern portion of the property contains the northeastward 
continuation of the Pennington-Armour No. 1 ore belt, while the north- 
ern portion contains the continuation of the Mahnomen ore belt. With 
the exception of a few schist bands, the entire 40-acre tract is underlain 
by iron-bearing rock and associated ore. Both iron ore and manganif- 
erous iron ore are present on the property. 

The shaft is a round timber shaft. It was sunk during the summer 
and fall of 1916 to a depth of 100 feet, of which the upper 52 feet are 
in glacial drift. 

KENNEDY MINE 

The shafts and principal underground workings of the Kennedy mine, 
operated by Rogers-Brown Ore Company, are located on the Orelands 
property, lot 5, section 30, T. 47N., R. 28W. The older underground 
workings, however, extend also into the SE*4 of SE^4 of section 30, 
known as the Brown property, and into lot 6 and the SW% of SW>4 of 
section 29, T. 47N., R. 28W., known respectively as the Harrison and , 
Ehric properties. 

The mine has a number of shafts, only two of which, however, are 
in use. The hoisting shaft, which is a wooden drop shaft, is located near 
the southeastern corner of lot 5, while the timber shaft is near the center 
of lot 5. The main hauling level is at 262 feet, being at the bottom of 
the main shaft, but the 150-foot level also connects with the main shaft. 
Besides these there are numerous sublevels. 

The mine is south of Rabbit Lake, at the northeastern end of a belt 
of iron-bearing rock which is known to extend southwestward a short 
distance beyond the southwestern end of Rabbit Lake, while to the north- 
east it runs under the lake. Traces of iron-bearing rock are found 
farther southwest along the same strike north and northwest of Menomin 
Lake, but their relation to the Kennedy belt is uncertain. The iron- 
bearing rock at the Kennedy mine, as indicated by the underground 



160 GEOLOGY OF EAST CENTRAL MINNESOTA 

operations, occupies a belt about 1400 feet wide including ore, ferrugi- 
nous chert, hematitic schist, and green chloritic schist, all of which occur 
in irregular bands having a general strike of N.45°E. This broad belt 
of iron-bearing rock and schist is bounded both on the northwest and 
southeast by areas of slate and schist, which near the contact of the 
iron-bearing rock is generally altered to paint rock. This alteration is 
characteristic also of the chloritic schist included within the iron-bearing 
beds. The various bands are the expression on a horizontal plane of 
steeply inclined beds and lenses of different rocks irregularly interlayered. 
The general dip of the rock layers varies in different parts from 55 to 
70 S.E. The hematitic schist and green chloritic schist within the iron- 
bearing beds are usually in the form of discontinuous lenses more or less 
parallel to the bedding of the iron-bearing rock. The hematitic schist 
usually occupies the borders of such lenses and extends for distances 
varying from a few feet to 50 feet or more into the interior, the unaltered 
centers being green chloritic schist. Some lenses are all of hematitic 
schist. 

Judging from secondary structures in different parts of the mine, 
it appears that the entire body of iron-bearing rock is a straight succes- 
sion. The beds appear to be on the south limb of an anticline, the older 
rocks occurring along the northwest side of the belt. The iron-bearing 
rock consists mainly of ferruginous chert with local irregular bodies of 
iron ore. In places the ferruginous chert is hard and massive, while 
elsewhere it is partly disintegrated forming wash ore. With depth, the 
ferruginous chert grades into cherty and argillaceous ferrous carbonate 
and silicate rocks such as cherty and slaty iron carbonate, amphibole- 
magnetite rock, and magnetitic slate. These unoxidized phases are espe- 
cially abundant in the deeper workings of the western and northwestern 
parts of the mine. Thus above the 210 foot level there is hardly a trace 
of ferrous rocks, the entire iron-bearing rock being oxidized, while on 
the 262-foot level the unoxidized phases are abundant. Ferruginous 
chert, however, is still abundant on the 262-foot level, especially 
directly underneath the ore bodies where downward circulation has prob- 
ably been facilitated by the greater porosity of the material. The meta- 
morphosed magnetitic phases of the original iron-bearing rocfc are more 
common in the western portion of the workings, the unaltered carbonate 
phases occurring in the central and northern parts. In the unoxidized 
parts of the iron-bearing formation in the lower levels, green chloritic 
schist occurs as it does in the upper levels, but its alteration to red hema- 
titic schist is much less marked. Usually the green schist is in direct con- 
tact with the ferrous rocks, and locally it is somewhat difficult to dis- 
tinguish underground between the argillaceous ferrous rocks and the 



GEOLOGY OF THE PRINCIPAL MINES 161 

green schist. In many parts of the mine the schist contains small dis- 
seminated crystals of carbonate, probably .calcite. In the green chloritic 
schist these crystals are fresh and transparent, but in the red hematitic 
schist they are decomposed and form scattered white spots. 

There is thus a general gradation in the Kennedy mine from ore, 
wash ore, ferruginous chert, and hematitic schist in the upper levels, to 
cherty and argillaceous iron carbonate, amphibole-magnetite rock, mag- 
netitic slate, and green chloritic schist in the lower levels. The occur- 
rence of the different rocks is necessarily somewhat irregular, green 
chloritic schist being fairly common in the upper levels and ferruginous 
chert being abundant in the lower levels ; nevertheless the general change 
is very significant. 

There are four main ore bodies at the Kennedy mine, which, although 
irregular, are roughly in the form of lenses parallel to the general trend 
of the rock layers. They are located in general along a line transverse 
to the bedding of the rock and have been designated, from northwest to 
southeast, the (i) north lens, (2) midway lens, (3) intermediate lens, 
and (4) south lens. The longer axis of each body is parallel to the 
bedding of the enclosing rocks. Little scattered lenses of ore are found 
between the main lenses and along their borders. The main shaft is 
located between the intermediate and south lenses. At the present time 
only the lenses north of the shaft are producing ore, but in the past con- 
siderable ore has been mined from the south lens. 

The ore from the different lenses varies somewhat as regards texture 
and composition. The characteristic ore of the north lens is distinctly 
laminated, dark reddish to bluish hematite associated with more or less 
brownish, siliceous, hydrated hematite and limonite. The latter becomes 
more abundant in the lower levels. The laminated ore is mostly amor- 
phous, but certain layers contain much crystalline hematite. The lamina- 
tion is caused by the interlayering of ore of different colors, or by the 
interlayering of siliceous laminae or crystalline laminae with the common 
amorphous ore. The ore usually breaks along lamination planes, often 
yielding large slabs. 

The principal ore of the midway lens is a soft, yellow to brown, 
ocherous limonite. Locally masses and fragments of hard, brown li- 
monite occur in it. Bedding is very indistinct and in places wanting. 

The intermediate and south lenses consist largely of ordinary hard, 
amorphous, dark brown limonite or hydrated hematite. Bedding is 
fairly distinct, but instead of the straight lamination shown by the 
north lens ore, the bedding planes are irregular and the ore is con- 
siderably fractured and crumpled. Much of the ore is of good grade, 
but much of it is siliceous, grading into ferruginous chert. There seems 
to be little regularity in the occurrence of the richer or leaner portions. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



They are irregularly intermixed. Even the boundary between the ore 
body and the enclosing ferruginous chert is indefinite. Where the ore 
lies against hematitic schist, however, as southeast of the intermediate 
lens, the boundary is sharp. 

The differences in the character of the ore lead one to suspect 
that there were distinct differences in the character of the original iron- 
bearing formation from which the various types of ore were derived. 

The following is an average cargo analysis for ore from the Ken- 
nedy mine : 136 

Average Cargo Analysis of Ore, Dried at 212 F., Shipped from the Kennedy 
Mine during the Season 1916 

Loss by 

Fe P SiO s Mn A1,0. CaO MgO S ignition 

Per cent 55.09 .241 11.867 - 222 1-736 302 .229 .034 6.09 

SOUTH RANGE MINES 

ADAMS MINE 

The Adams mine is the easternmost mine on the south range, being 
situated in the SWJ4 of NWJ4 of section 30, T. 46N., R. 28W. It was 
operated by Cuyler Adams during 1914, but has since been idle. 

The shaft is of concrete and has a depth of 207 feet, the upper 136.5 
feet being in glacial drift and the remainder in a dark green, coarsely 
crystalline diabase. The pump station is at the bottom of the shaft and 
is excavated in the diabase. From the pump station the main level drift 
passes first southeastward and then turns and runs in a general south- 
westerly direction along the ore body. For a short distance, the main 
drift is in diabase beyond which it penetrates in succession a layer of 
amphibole magnetite rock and magnetitic slate, a layer of green schist 
and then another layer of magnetitic slate and amphibole-magnetite rock 
of considerable thickness, before it encounters the ore body. The layers 
have a general strike a little north of east and dip steeply south. The 
magnetitic slate and green schist layers through which the drift passes 
therefore form the foot-wall of the ore body. Immediately in contact 
with the ore, the magnetitic slate is oxidized and is soft and clayey, but 
in general it is hard, dense, and finely laminated. The green schist is 
soft chloritic schist, usually finely crystalline but showing distinct 
cleavage. 

The diabase through which the shaft passes is dark green in color and 
consists of a ground-mass of fine-grained, dark green chlorite in which 
are abundant, long, narrow laths of pink feldspar. These feldspar 
phenocrysts range in length up to four fifths of an inch, but are rarely 



**• Lake Superior Iron Ore Association, op. cit., p. 10. 



GEOLOGY OF THE PRINCIPAL MINES 



163 



wider than one twentieth of an inch. They are scattered irregularly- 
through the rock, giving it a coarse, ophitic texture. Near the bed rock 
surface the diabase is light brownish green and thoroughly decomposed. 
On nearing the contact of the magnetitic slate along the main drift, the 
diabase loses its porphyritic texture and becomes very fine-grained. 
However, it retains a fine ophitic texture. This clearly indicates that 
the rock is intrusive. 

The band of iron-bearing rock next to the diabase is a dark-colored, 
locally almost black, laminated amphibole-magnetite rock. It is hard and 
dense and in places contains layers several inches thick consisting entirely 
of dark gray chert. With this are interlaminated thinner layers of black 
argillaceous rock rich in iron. Locally the chert layers contain abundant 
brownish fibrous amphibole. 

The chloritic schist is distinctly green in color, some being light and 
some dark. It appears to be of very uniform composition, consisting 
almost entirely of chlorite. The schistosity varies in different parts. In 
some places the rock has fine cleavage lines and elsewhere it is more 
massive. It is fine-grained throughout. 

The bed of iron-bearing rock between the chloritic schist and the ore 
body is of considerable thickness and consists of a mixture of magnetitic 
slate and amphibole-magnetite rock, the former being more abundant. 
In general the rocks are more finely laminated and less siliceous than the 
iron-bearing rock between the diabase and the chloritic schist. The 
typical magnetitic slate consists of interlaminated light-green, finely crys- 
talline amphibole and black, siliceous or argillaceous, fine-grained mag- 
netite. In some laminae amphibole and magnetite are intermixed and 
then form greenish black layers. The laminae vary in thickness, usually 
being very fine, but in places ranging up to half an inch thick. The rock 
cleaves readily along lamination planes. The proportion of chert or * 
quartz present is small in the magnetitic slate, and with increasing sili- 
ceous material and coarser layering the slate grades into amphibole- 
magnetite rock. Thin pyrite veins occur in both the magnetitic slate and 
amphibole-magnetite rock. 

The ore of the Adams mine varies greatly in character and appear- 
ance. Some of it is finely laminated and medium soft, while some is 
hard. In color it usually ranges from dark brown or red to black, the 
more hydrated ore being dark brown. Some of the finely laminated 
varieties, although of good grade, show a greenish or grayish tint, as if 
they contained much clay. The ore in general is of good grade. 

HOBART EXPLORATION SHAFT 

The exploration shaft of the Hobart Iron Company of Pickands, 
Mather and Company, is situated in the SW}4 of SE}4 of section 8, T. 



164 



GEOLOGY OF EAST CENTRAL MINNESOTA 



45N., R. 29W. It is about two miles east of Woodrow and is on the 
same general iron-bearing belt on which the Wilcox mine at Woodrow is 
located. Several minor breaks, however, occur in the belt between the 
two localities. 

The shaft has a depth of 120 feet, of which the upper 76 feet are in 
glacial drift. Its bottom is in magnetitic iron-bearing rock while a cross- 
cut 205 feet long running southward from it at the 114-foot level passes 
through 165 feet of mixed magnetitic iron-bearing rock and ore into 
green slate. The beds are very steep, having an average dip of about 
8o°SE. The green slate forms the hanging wall of the iron-bearing bed. 
The foot-wall is not shown in the workings. 

The iron-bearing bed contains a number of different kinds of rock. 
The shaft is mainly in finely laminated, black, magnetitic slate and lean 
magnetic ore. Much of this material, especially near the surface, is 
oxidized to a brown or brownish red, siliceous limonite. Near the bottom 
of the shaft, in the cross-cut, is a rather massive, fine-grained, dark green 
chlorite rock. Some of this is dense, but some of it contains small rhom- 
bic cavities from which crystals of carbonate have evidently been leached. 
Southward from the chlorite rock, the cross-cut passes through different 
kinds of black or dark green cherty and slaty iron-bearing rocks, some 
bands of which average over 50 per cent in metallic iron. In some of the 
beds, distinct layers of dark-colored chert appear, while other beds con- 
tain much green argillaceous rock. In general, however, the material 
is black, finely laminated, hard, lean ore. Much of it is slightly mag- 
netic, but much has been completely oxidized and hydrated to a dense, 
brownish black, siliceous limonite. A drift runs eastward from the cross- 
cut in this lean ore for a distance of 80 feet. 

The hanging wall slate is in part argillaceous and in part chloritic. 
Both phases show distinct cleavage, the chloritic variety being slightly 
less schistose. Both are dark green. 

Above the iron-bearing bed, lying on its eroded edges and overlain by 
glacial drift, are a few feet of dark red fragmental rock. This rock, 
which is penetrated by the shaft, consists largely of small pebbles or con- 
cretions of impure ferruginous and argillaceous material cemented to- 
gether by siliceous iron oxide. It is believed to be analogous to the fer- 
ruginous Cretaceous rock occurring locally in the Mesabi district. 

WILCOX MINE 

The Wilcox mine, operated by the Paterson Construction Company, 
is located in the SE l 4 of NW}4 of section 13, T. 45N., R. 30W., just 
south of the village of Woodrow, and about 6 miles north of east from 
Brainerd. The property embraces also several of the adjoining 40-acre 
tracts. 



GEOLOGY OF THE PRINCIPAL MINES 



The iron-bearing belt on which the mine is situated runs across the 
northern half of section 13 approximately N.6o°E., and dips about 65 
south. It consists mainly of ore, but a little ferruginous chert and paint 
rock are associated with the ore, especially along the borders of the belt. 

The iron-bearing rock is bounded both on the north and south by 
light green and gray slate. The hanging wall slate near the contact of 
the iron-bearing rock is decomposed, clayey, and of a bluish green color. 
In some places quartz veins occur in it and at others it is mixed with 
brown iron-stained clay. Quartz and clay are also found in the ore along 
the contact. 

The portion of the ore bed occurring along the hanging wall usually 
consists of ocherous yellow limonite, soft but distinctly bedded. This 
grades into yellowish and brownish black, medium hard limonite, which 
forms the main ore body. 

The foot-wall of the iron-bearing rock is a light-colored, greenish or 
grayish slate with marked schistosity but with little or no evidence of 
bedding. Near the ore body the slate becomes dark red, due to impreg- 
nation with hematite, and is known as paint rock. It is medium hard 
and shows cleavage similar to the green and gray slate. Locally traces 
of bedding are found in it, and at such points the cleavage and bedding 
appear to be parallel. The contact between the paint rock and ore is 
usually sharp. 

Both the ore and wall rocks contain a soft but compact, fine-textured, 
decomposed rock, also termed paint rock, which has the appearance of 
being intrusive. It occurs in masses which, although in general parallel 
to the strike of the enclosing rocks, are dike-like in character. The rock 
itself is light green where associated with green or gray slate, and dark 
red where associated with dark red slate or ore. When wet it is soft 
and clayey, but on drying it hardens. It does not show any cleavage and * 
has sharp contacts with the enclosing rocks. 

The Wilcox shaft is situated north of the iron-bearing belt in the 
slate which forms the foot-wall of the ore body. It has a depth of 230 
feet, of which the upper 95 feet are in glacial drift and the rest in green 
and gray slate. The main hauling level is at 200 feet, and the principal 
auxiliary level is at 125 feet. Most of the ore now mined is being taken 
from the 112-foot level. 

The ore, as already mentioned, is in part a yellow or brown, porous, 
ocherous ore and in part a brownish black, medium hard ore. Both types 
are limonitic. The following average cargo analysis of the ore is given 
by the Lake Superior Iron Ore Association. 137 



"Like Superior Iron Ore Association, op. at., p. n. 



io6 



GEOLOGY OF EAST CENTRAL MINNESOTA 



Averagt Cargo Analysis of Ore Dried at 212 F., Expected from th* Wilcox 
Mine for the Season 1917 

Loss by 

Fc P SiO, Mn A^O, CaO MgO S ignition 

Per cent 57.40 .25 6.00 .50 2.90 .45 .25 .03 8.70 

BRAINERD-CUYUNA MINE 

The Brainerd-Cuyuna mine, of the Brainerd-Cuyuna Mining Com- 
pany, is situated in the southern part of the city of Brainerd in the NE*4 
of of section 36, T. 45N., R. 31W. The property includes also 

several of the adjoining 40-acre tracts. The shaft, which is situated 
northwest of the iron-bearing belt in the foot-wall slate, has a depth of 
164 feet, of which the upper 90 feet are in glacial drift. A cross-cut runs 
southeastward from it through slate into the north ore body at the 154- 
foot level. At the bottom of the shaft, the slate is light green and gray, 
but near the ore body it is dark red due to impregnation by hematite. 
Southeast of the north ore body is the south ore body, separated from 
it by a thin bed of barren rock. 

The ore from the Brainerd-Cuyuna mine is yellowish and brownish 
limonite typical of the south range. It occurs in two lens-like bodies, 
the north lens and the south lens. The north lens consists mainly of soft 
yellow ocherous ore, while the south lens ore is darker and somewhat 
manganiferous. 

The iron-bearing belt has a general trend of about N.5o°E. in the 
vicinity of Brainerd, running from the southeastern part of section 30, 
T. 45N., R. 30W., through the northwestern corner of section 31 into 
the northeastern part of section 36, T. 45N., R. 31W., from where it con- 
tinues southwestward through most of section 36. The bed has a fairly 
steep southeasterly dip. 

BARROWS MINE 

The Barrows mine, from which ore was produced during 191 3 and 
1914 by the Virginia Ore Mining Company, of M. A. Hanna and Com- 
pany, is situated in the NWJ4 of SE*4 of section 10, T. 44N., R. 31W. 
It is the westernmost of the south range mines that has been operated. 

The main hoisting shaft has a total depth of 160 feet, of which the 
upper no feet are in glacial drift and the rest in dark green, massive, 
finely crystalline diabase which forms the hanging wall of the ore body. 
The main hauling level was at 160 feet and the upper working level at 
120 feet when the mine was in operation. 

The ore body has a general strike of about N.40°E., while the dip 
varies from 65 southeast to vertical. At a few places northwesterly 
dips are found. The main cross-cut on the 160-foot level runs north- 
westward from the shaft, cutting through the hanging wall diabase and 



GEOLOGY OF THE PRINCIPAL MINES 



167 



the ore body into the foot- wall rock which consists of a mixture of iron- 
bearing rock and ocherous clay. The contact between the ore and hang- 
ing wall diabase is very regular in strike but varies in dip between 65 
and 85 southeast. The ore body has a width of about 60 feet where the 
main cross-cut passes through it. The contact between the ore and the 
foot- wall rock dips about 65 south. 

The diabase is a massive, dark green, medium fine-grained rock with 
distinct ophitic texture. Most of it is hard and appears to be fresh, but 
some of it, especially that near the contact of the ore is soft and altered. 
The altered phase still retains the original texture to a large extent, but 
its color is brownish green and it appears to consist mainly of clay and 
ocherous hydrated iron oxide with perhaps some chlorite. Even some of 
the fresh looking rock has suffered considerable alteration to chlorite. 

The ore as a rule is finely laminated and varies in color from yellow 
to dark brown or black. Yellow laminae are interlayered with dark 
brown laminae, the latter predominating, while in places thin argillaceous 
reddish or greenish layers occur. Much of the ore is ocherous and has 
a porous texture, but much of it, although soft, is dense and compact. 
Layers of dark brown or bluish brown hard ore are interbedded with 
the soft ore. The bedding as a rule dips steeply to the southeast, but 
locally northwesterly dips are found. The contact between the ore and 
the diabase is nearly everywhere sharp and distinct, but the contact with 
the foot- wall rocks is less so. 

The foot- wall rock at the Barrows mine is largely of the variety 
usually termed paint rock by the miners. Paint rock includes many 
kinds of soft, argillaceous, iron-stained rocks some of which are massive 
and some schistose. The paint rock at the Barrows mine is similar to the 
massive paint rock at the Wilcox mine, and like it appears to be the de- 
composed phase of a fine-grained igneous rock. As it occurs in the 
ground it is soft, wet and clayey, but on drying it hardens and becomes 
dense and compact. It is fine-grained, breaks with even fracture, and 
varies in color from grayish green, to ocherous yellow, brown, or dark 
red, depending on the degree of oxidation and hydration. The first 
change appears to be one of oxidation from the green ferrous form to 
the dark red ferric form. The latter then becomes hydrated and turns 
brown and yellow. Most of the rock is even colored throughout, but 
some shows lamination due to color banding. The foot- wall paint rock 
occurs as a layer about 30 feet thick where penetrated by the main cross- 
cut. Beyond it to the northwest are interbedded iron-bearing rock and 
paint rock. The layering of the iron-bearing beds usually dips steeply 
to the southeast, but the contacts between iron-bearing rock and paint 
rock are more irregular. 



GEOLOGY OF EAST CENTRAL MINNESOTA 



The original nature of the paint rock is difficult to determine. Its 
general appearance is that of a decomposed igneous rock, yet it is not 
unlike a decomposed, dense, homogeneous slate. Its general parallelism 
in layering with the iron-bearing rock suggests that the two rocks were 
laid down at approximately the same time. However, the minor irregu- 
larities along the contacts indicate that there was not continuous deposi- 
tion. In general it seems more probable that the paint rock is of igneous 
origin, representing either contemporaneous volcanic flows or later in- 
trusive sheets. 



BIBLIOGRAPHY OF EAST CENTRAL MINNESOTA AND THE 
CUYUNA IRON-ORE DISTRICT 

Adams, F. S. The iron formation of the Cuyuna range: Econ. Geology, vol. 5, 

pp. 729-40, 1910; vol. 6, pp. 60-70 and 156-80, 1911. 
Anon. A difficult shaft sinking operation at Deerwood, Minn. : Iron Tr. Rev., 

vol 43. PP- 772-74, Nov. 5, 1908. 
The Cuyuna range, Minnesota: Eng. and Min. Jour., vol. 90, p. 1214, Dec. 

17, 191a 

Cujuna range development: Eng. and Min. Jour., vol. 91, p. 1146, June 

xo f 191 1. 

Cuyuna's first ore shipment: Min. Wld., vol. 34, p. 1187, June 10, 191 1. 

Cuyuna range notes: Eng. and Min. Jour., vol. 92, p. 304, Aug. 12, 191 1. 

Heavy movement from Hill ore lands : Iron Tr. Rev., vol. 50, pp. 562-65, 

Mar. 7, 1912. 

Cuyuna iron range: Eng. and Min. Jour., vol. 93, p. 1025, May 25, 1912. 

Iron-ore railroads at Lake Superior: Eng. and Min. Jour., vol. 93, p. 1188, 

June 15, 1912. 

Cuyuna iron ores: Eng. and Min. Jour., vol. 93, p. 1284, June 29, 1912. 

First Cuyuna range stripping: Iron Tr. Rev., vol. 51, p. 1177, Dec. 19, 1912. 

The Cuyuna iron range: Eng. and Min. Jour., vol. 94, pp. 97-98, July 

20, 1912. 

Sinking a concrete shaft lining through quicksand: Eng. Rec, vol. 67, 

PP. 383-S4, April 5, 1913. 
Handling ore from the Cuyuna range: Iron Tr. Rev., vol. 53, pp. 729-33, 

Oct 23, 1913. 

Duluth, Minnesota: (Special correspondence on the Minnesota iron 

ranges). Min. and Sci. Press, vol. 108, p. 190 (Jan. 24), p. 469 (Mar. 14), 

p. 667 (Apr. 18), p. 1026 (June 20), 1914. 

The truth about the Cuyuna : Iron Tr. Rev., vol. 55, p. 972, Nov. 26, 1914. 

Quietus on speculators is placed by Minnesota Tax Commission, which 

develops the facts about the iron ore deposits of the Cuyuna range: Iron 

Tr. Rev., vol. 55, p. 985, Nov. 26, 19 14. 
Lake Superior iron ranges: Eng. and Min. Jour., vol. 99, pp. 73-76, Jan. 

9, 1915- 

Stripping the Pennington mine: Excav. Eng., vol. 11, pp. 175-76, Feb. 19, 

1915. 

Increased activity on the Cuyuna: Iron Tr. Rev., vol. 57, p. 191, July 22, 

I9I5- 

Good results follow careful work. South range on the Cuyuna promises 

to be lively district: Iron Tr. Rev., vol. 57, pp. 363-64, Aug. 19, 1915. 

Cuyuna range tonnage for 1915: Eng. and Min. Jour., vol. 100, p. 403, 

Sept 4, 1915. 

Electrical power for the Cuyuna range: Eng. and Min. Jour., vol. 100, 

pp. 403-4, Sept 4, 1915. 
The Dean-Itasca and Pennington mines : Iron Tr. Rev., vol. 57, pp. 1 140-41, 

Dec 9, 1915. 

Armour mines on Cuyuna range : Iron Tr. Rev., vol. 57, pp. 1223-24, Dec 

23. I9I5« 



170 



BIBLIOGRAPHY 



Anon. Stripping the Hillcrest mine with a sand pump: Eng. and Aft*. Jour., vol. 

101, pp. 211-15, Jan. 29, 1916. 
Welfare work on the Cuyuna range: Iron Tr. Rev., vol. 58, p. 450, Feb. 

24, 1916. 

Method of drop-shaft sinking on Cuyuna range: Eng. and Ifim. Jour., 

vol. 101, p. 404, Feb. 26, 1916. 
The Rowe mine ore-washing plant, Minnesota: Min. IVld., tel. 44, pp. 

517-19, Mar. 11, 1916. 
Predicted troubles not realized at the Adams mine on the Cuyuna: Iron 

Tr. Rev., vol. 58, p. 1284, June 8, 1916. 
Appleby, W. R., and Newton, E. Preliminary concentration tests cm Cuyuna 

ores: Univ. of Minn. School of Mines Exp. Sta. Bull. No. 3, 1915. 
Bark, J. C. Chain grizzly at the Rowe mine: Eng. and Min. Jour., vol. iox, pp. 

599-600, Apr. 1, 1916. 
Barrows, W. A., Jr. The Cuyuna district, South range, iron ores : Irom Tr. Rev., 

vol. 51, pp. 923-24, Nov. 14, 1912. 

(See also Zapffe, Carl, and Barrows, W. A., Jr.) 
Berkey, C. P. Geology of the St. Croix dalles: Amer. Geol., vol. 20, p». 345-83, 

1897; vol. 21, pp. 139-55 and 270-94, 1898. 
Bradt, H. H. (See Newton, E., and Bradt, H. H.) 

Burchard, E. F. Iron ore, pig iron, and steel : Min. Res. U. S., U. S. Geol. Sur- 
vey. Reports for 1911, 1912, 1913, 1914, 1915, and 1916. 

Cheney, C. A., Jr. Structure of the Cuyuna iron-ore district of Minnesota : 
Eng. and Min. Jour., vol. 99, pp. 1113-15, June 26, 1915. 

Crowell and Murray. The iron ores of Lake Superior: pp. 159-61, 1914. 

The iron ores of Lake Superior: pp. 201-212, 1917. 

Croze, W. W. J. Minnesota's great iron-mining industry: Min. Wld., vol. 33, pp. 
717-21, Oct. 15, 1910. 

Donovan, P. W. Churn drill angle holes on the Cuyuna : Eng. and Mm. Jour., 

vol. 96, pp. 1 1 17-18, Dec. 13, 1913. 
Some aspects of exploration and drilling on the Cuyuna range : Proc. 

Lake Sup. Min. Inst., vol. 20, pp. 136-41, Sept. meeting, 1915. Also Iron Tr. 

Rev., vol. 57, P- 534, I9I5- 
Edwards, G. E. Rapid development of the Cuyuna range: Min. Wld., vol. 39, pp. 

339-41, Aug. 23, 1913. 
Mining activity on the iron ranges: Min. Wld., vol. 43, pp. 353-60, Sept. 

4, 1915. 

Emmons, W. H. Outline of the geology of the iron ranges. In Iron mining in 
Minnesota, by C. E. van Barneveld, Univ. of Minn. School of Mines Exp. Sta. 
Bull., No. 1, pp. 12-23, 1912. 

Estep, H. C. Independents are conspicuous. A review of interesting develop- 
ments of the past year on the Minnesota iron ranges : Iron Tr. Rev., vol. 56, 
PP. 73-8i, Jan. 7, 1915. 

Iron range developments in 1915 : Iron Tr. Rev., vol. 58, pp. 81-92, Jan. 

6, 1916. 

Gregory, W. Bibliography of Minnesota mining and geology: Univ. of Minn. 

School of Mines Exp. Sta. Bull. No. 4, pp. 105-6, 1915. 
Grout, F. F. Contribution to the petrography of the Keweenawan : Jour. Geology, 

vol. 18, pp. 633-57, 1910. 
Hall, C. W. Keweenawan area of eastern Minnesota: Bull. Geol. Soc. Amer., 

vol. 12, pp. 313-42, 1901. 



BIBLIOGRAPHY 



171 



Keewatin area of eastern and central Minnesota: Bull. Geol. Soc. Amer., 

vol. 12, pp. 343-76, 1901. 
Harder, E. C. Manganese deposits of the United States with sections on foreign 

deposits, chemistry, and uses : U. S. Geol. Survey Bull. 427, pp. 130-31, 1910. 
Manganiferous iron ores of the Cuyuna district, Minnesota: Amer. Inst. 

Min. Eng. Bull, Sept. 1917. 

Manganiferous iron ores : U. S. Geol. Survey Bull. 666-EE, 1917. 

and Johnston, A. W. Notes on the geology and iron ores of the Cuyuna 

district, Minnesota: U. S. Geol. Survey Bull. 660-A, 1917. 
Hewett, D. F. Manganese and manganiferous ores: Min. Res. U. S., U. S. Geol. 

Survey, Reports for 1913, 1914, 1915, and 1916. 
Hurd, Rukard. Iron ore manual of the Lake Superior district, pp. 41-42, and 

75-77, 191 1. 

Irving, R. D. The geological structure of northern Wisconsin : Wis. Geol. and 

Nat. Hist. Survey, Geol. of Wis., vol. 3, 1873-1879, p. 18, 1880. 
The copper-bearing rocks of Lake Superior : U. S. Geol. Survey Third 

Ann. Rept., p. 162 and PI. Ill, 1883. 
Preliminary paper on an investigation of the Archean formations of the 

northwestern states : U. S. Geol. Survey Fifth Ann. Rept., pp. 196-97 and PI. 

22, 1885. 

Classification of early Cambrian and pre-Cambrian formations: U. S. 

Geol. Survey Seventh Ann. Rept., pp. 417-23 and PI. 41, 1888. 
Kellogg, L. O. Notes on the Cuyuna range: Eng. and Min. Jour., vol. 96, pp. 

1199-1203, Dec. 27, 1913; vol. 97, PP- 7-io, Jan. 3, 1914. 
Lake Superior iron ranges : Eng. and Min. Jour., vol. 97, pp. 83-86, Jan. 

10, 1914. 

Stripping with the hydraulic giant: Eng. and Min. Jour., vol. 97, pp. 

166-67, Jan. 17, 1914. 

Stripping the overburden in open pit mining : Eng. Mag., vol. 50, pp. 

896-909, Mar. 1916. 

Kloos, J. H. Geological notes on Minnesota : Geol. and Nat. Hist. Survey of 
Minn. Tenth Ann. Rept., pp. 175-200, 1881. Also Zeitschr. d. Deutsch. Geol. 
Geseli, p. 417, 1880. 

(See also Streng, A., and Kloos, J. H.) 

Johnston, A. W. (See Harder, E. C, and Johnston, A. W.) 

Lake Superior Iron Ore Association. Analyses of Lake Superior iron ores : Year- 
books for seasons 1911-1917. 

Leith, C. K. A summary of Lake Superior geology with special reference to the 
recent studies of the iron-bearing series : Trans. Amer. Inst. Min. Eng., vol. 
36, pp. 116 and 149-50, I9d6. 

Geology of the Cuyuna iron range, Minnesota : Econ. Geology, vol. 2, pp. 

145-52, 1907. 

(See also Van Hise, C. R., and Leith, C. K.) 

McAuliffe, P. J. Stripping a mine by hydraulic methods: Coal Age, vol. 5, 
pp. 568-69, Apr. 4, 1914. 

McCarty, E. P. Hydraulic stripping at Rowe and Hillcrest mines on the Cuyuna 
range, Minnesota: Proc. Lake Sup. Min. Inst., vol. 20, pp. 162-73, Sept. meet- 
ing, 1915. 

Manganiferous iron ores of the Cuyuna range: Eng. and Min. Jour. t vol, 

100, pp. 400-2, Sept. 4, 1915. 



BIBLIOGRAPHY 



McCabty, E. P. Hydraulic stripping on Cuyuna : Iron Tr. Rev., vol. 5^, PP- 135-30. 
Jan. 13, 1916. 

Minnesota Bureau of Labor. Reports of mine inspectors. 14th Biennial Rept., pp. 

145 et seq., 1913-1914- 
Minnesota Tax Commission 

First Biennial Rept., 1908, pp. no et seq. 

Second Biennial Rept., 1910, pp. 49 et seq. 

Third Biennial Rept., 1912, pp. 58 et seq. 

Fourth Biennial Rept., 1914, pp. 75 et seq. 

Fifth Biennial Rept., 1916, pp. 73 et seq. 
Neimeyer, C. The Cuyuna iron range: Eng. and Min. Jour., vol. 91, p. 797, 

Apr. 22, 191 1. 

Newton, E. Concentration of Cuyuna ores: Proc. Lake Sup. Min. Inst., vol. 20. 

pp. 200-12, Sept. meeting, 1915. 
Manganiferous iron ores of the Cuyuna district: Univ. of Minn. School 

of Mines Exp. Sta. Bull. 5, 1918. 
and Bradt, H. H. Beneficiation of Lake Superior ores. In iron 

ore, pig iron, and steel, by E. F. Burchard: Min. Res. U. S., 1915, U. S. Geol. 

Survey 1916. 

(See also Appleby, W. R., and Newton, E.) 
Rotthaus, J. E. Magnetic surveying on the Cuyuna: Eng. and Min. Jour., vol. 98, 

pp. 603-4, Oct. 3, 1914. 
Sawhill, R. V. 1915 Lake Superior ore shipments : Iron Tr. Rev., vol. 58, pp. 

602-5, Mar. 16, 1916. 
1916 Lake Superior ore shipments : Iron Tr. Rev., vol. 60, pp. 535-37, 

Mar. I, 1917. 

Spurr, J. E. The stratigraphic position of the Thomson slates : Amcr. Jour. Sci., 

3d ser., vol. 148, pp. 159-66, 1894. 
Streng, A., and Kloos, J. H. The crystalline rocks of Minnesota : Geol. and Nat. 

Hist. Survey of Minn. Eleventh Ann. Rept., pp. 30-85, 1882. Also Neues Jahrb. 

f. Min. u. Petrifactenkunde, p. 36, 1877. 
Swanson, August. Annual report of Inspector of Mines, Crow Wing County, 

1st, 2d, and 3d reports, June 1913, June 1914, and June 1915. 
Sweet, E. T. Geology of the western Lake Superior region : Wis. Geol. and Nat. 

Hist. Survey, Geol. of Wis., vol. 3, 1873-1879, pp. 334 et seq., 1880. 
Thomas, Kirby. Notes on the geology of a new iron district in Minnesota: Mines 

and Minerals, vol. 25, p. 27, Aug. 1004. 
■ A new, promising Lake Superior iron district: Min. Wld., vol. 21, pp. 

446-48, Nov. 5, 1004. 

Non-Bessemer ore will probably be principal product of the Cuyuna range : 

Iron Tr. Rev., vol. 50, p. n 78, May 30, 1912. 
The Cuyuna iron range: Min. and Sci. Press, vol. 105, pp. 52-53, July 13, 

1912. 

Thwaites, F. T. Sandstones of the Wisconsin coast of Lake Superior: Wis. 
Geol. and Nat. Hist. Survey Bull. 25, 1912. 

Upham, Warren. Notes of rock outcrops in central Minnesota: Geol. and Nat. 
Hist. Survey of Minn. Eleventh Ann. Rept., pp. 86-136, and 220, 1884. 

Preliminary report of field work during 1893 in northeastern Minnesota, 

chiefly relating to glacial drift: Geol. and Nat. Hist. Survey of Minn. Twenty- 
second Ann. Rept., pp. 26-29, 1894. 



BIBLIOGRAPHY 



175 



— . Geology of Benton, Sherburne, Stearns, Todd, Wadena, Crow Wing, 

Morrison, Mille Lacs, Kanabec, and Pine counties: Geol. and Nat. Hist. Sur- 
vey of Minn., Geol. of Minn., vol. 2, 1882-1885, pp. 426-70 and 562-645, 1888. 

Geology of Aitkin, Cass, and Crow Wing counties: Geol. and Nat. Hist. 

Survey of Minn., Geol. of Minn., vol. 4, 1896-1898, pp. 25-81, 1899. 

Van Barneveld, C. E. Iron mining in Minnesota : Univ. of Minn. School of Mines 
Exp. Sta. Bull. 1, pp. 205-7, 191 2. 

Van Hise, C. R. Correlation papers : Archean and Algonkian : U. S. Geol. Survey 
Bull. 86, pp. 51-208, 1892. 

and Leith, C. K., Pre-Cambrian geology of North America: U. S. Geol. 

Survey Bull. 360, pp. 108-402, 1909. 

The geology of the Lake Superior region: U. S. Geol. Survey Mon. 

52, pp. 211-24 (Chap. IX, Cuyuna district, etc., Carl Zapffe, joint author), 
191 1. 

Winchell, N. H. Note on the age of the rocks of the Mesabi and Vermilion iron 
districts : Geol. and Nat. Hist. Survey of Minn. Eleventh Ann. Rept., pp. 168-70, 
1882. 

The crystalline rocks of the northwest: Geol. and Nat. Hist. Survey of 

Minn. Thirteenth Ann. Rept., pp. 124-40, 1884. Also Proc. Amer. Assn. Adv. 

Sci., 33d meeting, pp. 366-79, Sept. 1884. 
The crystalline rocks of Minnesota : Geol. and Nat. Hist. Survey of Minn. 

Thirteenth Ann. Rept., pp. 36-38, 1884. 
The crystalline rocks of Minnesota : Geol. and Nat. Hist. Survey of Minn. 

Seventeenth Ann. Rept., pp. 5-74* 1888. 
The crystalline rocks: Geol. and Nat. Hist. Survey of Minn. Twentieth 

Ann. Rept., pp. 1-28, 1891. 
The pre-Silurian rocks of Minnesota: Geol. and Nat. Hist. Survey of 

Minn. Twenty-first Ann. Rept., Table opposite p. 4, 1892. 
The geology of Carlton and southern St. Louis counties: Geol. and Nat. 

Hist. Survey of Minn., Geol. of Minn., vol. 4, 1896-1898, pp. 1-24 and 212-21, 

1899. 

'— Geological map of Minnesota: Geol. and Nat. Hist. Survey of Minn., Geol. 

of Minn., vol. 6, 1900-1901, Frontispiece, 1901. 

The Cuyuna iron range: Econ. Geology, vol. 2, pp. 565-70, 1907. 

Woodbridge, D. E. Electric power in Mesaba mining: Iron Age, vol. 75, pp. . 

2069-70, June 29, 1905. 
Iron ore in Crow Wing County, Minn. : Eng. and Min. Jour., vol. 84, pp. 

775-76, Oct. 26, 1907. 
Lake Superior iron mines in 1907: Eng. and Min. Jour., vol. 85, p. 114, 

Jan. 11, 1908. 

Notes from the Lake Superior iron ranges: Eng. and Min. Jour., vol. 89, 

pp. 863-64, Apr. 23, 1910. 
The Lake Superior iron country: Eng. and Min. Jour., vol. 91, p. 31, 

Jan. 7, 191 1. 

Iron ore in Lake Superior district: Eng. and Min. Jour., vol. 93, p. 31, 

Jan. 6, 1912. 

The Lake Superior iron-ore district: Eng. and Min. Jour., vol. 95, p. 85, 

Jan. 11, 1913. 

Changes and outlook in the Lake iron-ore trade : Iron Age, vol. 93, pp. 15-18, 

Jan. 1, 1914. 



174 



BIBLIOGRAPHY 



Zapffe, Caul. The geology of the Cuyuna iron-ore 'iistrict Unpublished thesis, 
Univ. of Wis., 1907. 

The Cuyuna iron-ore district of Minnesota : Brainerd Tribune, Supplement, 

Sept. 2, 1910. 

Geology of Cuyuna iron-ore district, Minnesota: Min. Wld., vol. 34, pp. 

585-88, Mar. 18, 191 1. 

Geology of the Cuyuna iron-ore district, Minnesota : Science, n. s., vol. 33, 

p. 463, Mar. 24, 191 1. 

The Cuyuna iron range: Eng. and Min. Jour., vol. 91, p. 993, May 20, 191 1. 

South range of the Cuyuna district: Brainerd Daily and Weekly Dispatch 

Supplement, Jan. 19, 1912. 

A survey of the developments and operations in the Cuyuna iron-ore dis- 
trict of Minnesota: Proc. Lake Sup. Min. Inst., vol. 20, pp. 125-35, Sept. meet- 
ing, 1915. 

Matters of interest to operators regarding the Cuyuna district: Proc. Lake 

Sup. Min. Inst., vol. 20, pp. 191 -99, Sept. meeting, 1915. 
Development of the Cuyuna range: Iron Tr. Rev., vol. 57, pp. 1 131-33 

and 1154b, Dec. 9, 1915. 
and Barrows, W. A., Jr. The iron ores of the South range of the Cuyuna 

district, Minnesota: Trans. Amer. Inst. Min. Eng., vol. 44, pp. 3-13, 1912. 

Abstract, Iron. Tr. Rev., vol. 51, p. 881, Nov. 7, 1912. 

(See also Van Hise, C. R., and Leith, C. K.) 



INDEX 



Page 

Adams, Cuyler 97 

work of 97 

Adams, F. S., work of 99 

Adams mine 104 

analysis of ore from 120 

geology of 162 

production of 108 

Agawa formation, nature of 11 

Aitkin County, rock outcrops in 45,52,64 

Algoma mine 102 

geology of 137 

production of . 108 

Altitudes 95 

Amphibole, occurrence of 60 

Amphibole-magnetite rocks, analysis of . . . . 126 

composition of 4, 117 

correlation of . 117 

Animikie group, distribution and charac- 
ter of ii, 109, 123 

Ann River, granite outcrops near 43 

Archean system, rocks of 10 

Argillite, term defined 3,4 

Armour No. 1 mine 100 

geology of 144 

production of 108 

Armour No. 2 mine 101 

geology of 146 

production of 108 

Atkinson, outcrops near 83 

Barnum, outcrops near 76 

Barrows mine 102 

production of 108 

Bayfield group, correlation of 13 

Benton County, rock outcrops in 38, 40, 63 

Berkey, C. P., referred to 28 

Biwabik formation, distribution and char- 
acter 12 

Black Hoof River, rock outcrops near.... 85 
Blackhoof Lake, analysis of ore from.... 120 

Bowles, Oliver, referred to 39 

Bradbury Brook, granite outcrops near.... 41 
Bradt, H. H., acknowledgments to.... 155, 156 

Brainerd-Cuyuna mine 105 

geology of 166 

production of 108 

Brainerd Mining Company 103 

Brookston, slate near 90 

Cambrian system, rocks assigned to 9, 91 

Canadian-Cuyuna Iron Mining Company.. 105 

Carbonate rock, analyses of 120 

occurrence and character of 3, 116 

relation to iron ore deposits 130 

Carlton, rock outcrops near 90 

Carlton County, rock outcrops in. .54, 67, 75, 92 

Cass County, granite outcrops in 30,31 

Cheney, C. A., referred to 114 

Chert, distribution and character of 115 

nature of 3 



Page 

Chlorite, occurrence of 60 

Chloritic schist, analysis of 120 

distribution and character of 58, no 

occurrence of "4 

origin of "9. 122 

Clements, J. M., referred to 10, 21 

Cloquet, rock outcrops near 90 

Coldspring, rock outcrops near 39 

Concentrating plant at Rowe mine 104 

j Cowan Brook, granite outcrops near 44 

Cretaceous rocks, distribution and char- 
acter of 14, 30, 93. 94. "0» I2 4 

nature of 8 

Croft mine 107 

geology of !5o 

production of 108 

I Crow Wing County, iron ore in 125 

limestone in 63 

j Cowan Brook, granite outcrops near 44 

Cuyuna district, area of 95 

geology of 107 

history of 96 

location of 94 

Cuyuna-Mille Lacs mine 102 

geology of 152 

jasper in 118 

production of 1Q 8 

I Dam Lake, quartzite outcrops near 52, 64 

j Deerwood members, structure of 114 

Denham, rock outcrops near .47, 65 

Drainage 95 

Duluth-Brainerd Iron Company 105 

Feigh property 142 

Ferguson, H. G., work of 135, ^37, 141. *57 

Ferro mine 105 

geology of 135 

production of 108 

; Ferruginous chert, analyses of 120 

nature of 115, 129 * 

term defined 3 

■ Ferruginous slate, analyses of 120 

distribution and character of 116 

nature of 3 

Fishtrap Creek, rock outcrops near 49 

Folding, extent of in 

Freedham, granite outcrops near 32 

Fuller, M. L., work of 5,8,9.14 

Garnet, occurrence of 60,62 

Geological investigation, previous 15 

i Giants Range granite, correlation of.... 30, 31 

Gillespie Brook, rock outcrops near 69 

Glacial deposits, distribution and charac- 
ter of 5. i©7 

I Gneiss, distribution and character of 46 

Granite, distribution and character of 30 

1 Granite City, granite outcrops near 35 

: Grant, U. S., referred to 6, ai 

I Gravelville, granite outcrops near 33 



INDEX 



Greenstone, distribution and character of. . 10 

Grout, F. F., referred to 13,38,35,42,58 

Hall, C. W. referred to 

5. 8, 9, U. *3. «8, 29, 63, 93 

Hanna, M. A., and Company 98, 166 

Harrington, M. W., referred to 5 

Hay Creek, granite outcrops near 44 

Hematite, distribution and character of. . 

J33, 143. «47, 161 

Hrmatitic chert, distribution and charac- 
ter of 115 

nature of 3 

Hematitic schist, analysis of iao 

occurrence of 119 

origin of 4 

Hematitic slates, analysis of 100 

distribution and character of 115 

nature of 3 

Hill Mines Company 107, 141 

Hillcrest mine 106 

geology of 141 

production of 108 

Hillman Creek, granite outcrops near. .34, 36, 37 
Hinckley sandstone, distribution and char- 
acter of 93 

explanation of term 91 

History of district 96 

Hobart exploration shaft 163 

Hobart Iron Company 98, 163 

Hoch mine 102 

Hopkins mine, geology of 157 

Hornblende schist, distribution and char- 
acter of 66, 72 

Huronian series, distribution and charac- 
ter of 11, 16 

Huronian, upper. See Animikie 

Hydraulic stripping at Rowe mine 103 

Igneous rocks, lithology of 112 

silicic, outcrops of 30 

subsilicic, outcrops of 49 

Inland Steel Company. .98, 100, lot, 144, 146, 148 
Intrusive rocks, distribution and charac- 
ter of 10, 123 I 

See also Igneous rocks 
Iron-bearing formation, analysis of speci- 
mens of 120 

association of, with igneous rocks.... 119 

composition of 113 

deposition of 130 

distribution and character of 114, 

118, 124, 140, 159. 163, 164, 165 

horizons of 113 

term defined 3 

See also particular formations 
Iron carbonate rock. See Carbonate rock 

Iron Mountain mine 102 

Iron Mountain Mining Company 102 

Iron ores, occurrence and character of.... 

118, 129, 132 

origin of 130 

production of 107, 108 

relation to adjacent rocks 13a 



Pat* 

Ironton mine torn 

geology of 146 

production of 108 

Iroquois Iron Company 100, 101 

Irving, R. D., referred to 15,16 

Jispilite, definition of 3 

distribution and character of 118 

Joan mine, geology of 157 

Joan No. 3 mine, geology of 137 

Jones and Laughlin Steel Company 98 

Kanabec County, granite outcrops in 43 

sandstone in 91 

Keeweenawan series, correlation of 110 

distribution and character of 

13. 93. Jio, 134 

folding in 1 1 j 

intrusions of 123 

sediments in 13 

Kellogg, L. O., referred to 96 

Kennedy mine 99, 100 

analyses of rock from 120 

geology of 159 

production of 108 

Kettle Lake, slate near 68 

Kettle River, rock outcrops near 56,68 

sandstone near 91 

Knife Lake slate, distribution and charac- 
ter of 11 

Lake Emily, iron bearing rocks near 125 

Lake Superior Iron Ore Association. Anal- 
yses of Lake Superior iron 

ores. Quoted 137, 138, 

142, 144, US. M6, 148, 149, 

150, 151, 153. 156. *S8, 162, 165 

Lang, W. B., acknowledgments to 2 

Lawson, A. C, referred to 18 

Ledoux, rock outcrops near 61 

Leech Lake, granite outcrops near 31 

Leith, C. K., referred to 

6, 10, 13, 21, 22, 24, 

35. J«i 53. 63. 109, lit, 124, 129 
Limestone, distribution and character of. . 66 
Limonite, distribution and character of.... 134 
Limonitic chert, distribution and charac- 
ter of 115 

nature of 3 

Limonitic slates, distribution and charac- 
ter of 115 

nature of 3 

Little Elk River, rock outcrops near 30 

Little Falls, rock outcrops near 

27, 3», SO, 60, 61 

Long Lake, rock outcrops near 3* 

McAuliffe, P. J., referred to 103 

McCarthy, J. H., analyses by iao 

McCarty, E. P., referred to 103 

McGrath, rock outcrops near 46 

Magnetic surveys 97 

Magnetitic slates, composition of 3. »*7 

correlation of "7 



INDEX 



177 



Page 

Mahnomen mine 106 

geology of 153. *57 

production of 108 

Mahtowa, outcrops near 77, 79 

Malloy dam, granite outcrop near 46 

Mangan No. 1 mine 106 

geology of ISA 

production of 108 

Mangan No. 2 mine 106 

analyses of rock from 120 

geology of 159 

production of 108 

Manganese ore, occurrence of 134 

Manganiferous ore, distribution and char- 
acter of 118,125,135,156, 158 

nature of 119. 134 

production of 107, 108 

Manitou eruptives 93 

Meacham mine 107 

geology of 150 

production of ; . 108 

Meinzer, O. E., referred to 5, 8, 9, 14 

Melrose, rock outcrops near 31 

Merrimac Mining Company 107 

Mesabi district, geology of 11 

geological report on 20,21 

Metamorphic rocks, outcrops of 58 

Metamorphism, effects of 63, 124 

Meyer's quarry 34 

Mica schist, distribution and character of. . 67 
Michalski, William, rock outcrop on land of 56 

Mill Island, rock outcrops on 60 

Mille Lacs County, granite outcrops in 40 

Mille Lacs Mine. See Cuyuna-Mille Lacs 
mine 

Minneapolis, St. Paul, and Sault Ste. 

Marie Railroad. Cuyuna Range 

Branch 96, 100 

Minnesota, east central, sequence of rocks 

in 24 j 

Minnesota, northeastern, sequence of rocks 

in 6 

Minnesota, southern, sequence of rocks in 5 
Minnesota Geological and Natural History 

Survey, early investigations of 18 j 

rock classification used by 19 

Minnesota University. School of Mines 

Experiment Station, analyses 

made by 119 

Moose Horn River, rock outcrop near.... 76 i 

Moose Lake, rock outcrops near 75 ; 

Moose River, rock outcrops near 79 ( 

Morrison County, chloritic schists in 58 

iron-bearing formation in 125, 128 

rock outcrops in 32, 50 

Muscovite, occurrence of 66 

Newton, Edmund, acknowledgments to. .155, 156 
Northern Pacific Railroad. Duluth-Brain- 

erd branch 96 

Ogelbay, Norton and Company 98 

Ogishke conglomerate, distribution and 

character of 10 



Page 

Oliver Mining Company 98,90 

Onahman Iron Company 106, 135 

Onamia, granite outcrops near 40,42 

Ore analyses 

Algoma mine 138 

Armour No. 1 mine 145 

Armour No. 2 mine 148 

Croft mine 151 

Cuyuna-Mille Lacs mine 154 

Ferro mine 137 

Hillcrest mine 142 

Ironton mine 146 

Kennedy mine 162 

Mahnomen mine 159 

Meacham mine 150 

Pennington mine 144 

Sultana mine 156 

Thompson mine 149 

Wilcox mine 166 

Otter Creek, rock outcrops near 87 

Ottrelite, occurrence of 60 

Paint rock, distribution and character 

of 165, 167 

nature of 168 

Paleozoic rocks, distribution and character 

of 14 

Paragenesis of garnetiferous and staurolitic 

schist 63 

Park Lake, rock outcrops near 81 

Park Lake River, rock outcrops near. . . . 57, 78 

Paterson Construction Company 105, 164 

Pennington mine 101 

geology of 142 

production of 108 

Peterson, J. P., slate on land of 80 

Pevon, L., diabase outcrop on land of.... 54 

Phyllite, definition of 4 

distribution and character of 59, 72 

Pickands, Mather and Company 98, 163 

Pierz, granite outcrops near 34 

Pine County, rock outcrops in 47. 57. 6s 

sandstone in 91 

Pine River, rock outcrops near 47 

Pittsburgh Steel Ore Company 103, 139 

Potsdam sandstone 91 

Puckwunge conglomerate 93 

Pyrite, distribution and character of 78 

Quartzite, distribution and character of. .66, 122 
Quartzitic mica schist, distribution and 

character of 65 

Randall, chloritic schists near 59 

Ravicz, L. G., acknowledgments to 2 

Red clastic series, correlation of 9 

distribution and character of 9 

Rocks, age of 109 

classification used by Minnesota and 
United States surveys com* 

pared 18 

distribution and character of 23 

sequence of 5,6,17 

Rockville, granite outcrops near 39 



7H 



WELLESLEY COLLEGE LIBRARY 



3 5002 03596 5222 



Page 

Rogers-Brown Ore Company. .99, 101, 104, 159 

Rowe mine 103 

analysis of ore from 120 

concentration plant at 104 

geology of 139 

production of 108 

Rucker, granite outcrops near 35 

Rum River, granite outcrops near 40,42 

Saint Cloud, granite outcrops near 39 

St. Croix River, lava near 28 

rock outcrops near 13,14.30 

sandstone near 92 

St. Croix sandstone, distribution and char- 
acter of 29 

explanation of term as used by the 

Minnesota geologists 92 

Saint Francis River, granite outcrops near 40 

St. Louis River, rock outcrops near 

57. 88, 9'. 92 

St. Louis slate, correlation of 16,20 

Sandstone, distribution and character of. . 90 

Sauk Center, rock outcrops near 31 

Sauk Rapids, granite outcrops near 38 

Savage, John A., and Company 107, 150 

Schultz, A. R., referred to 5 

Sherburne County, granite outcrops in.... 39 
Silicic igneous rocks, distribution and char- 
acter of 30 

Silver Creek, rock outcrop near 69 

Sioux quartzite, correlation of 9 

distribution and character of 8 

Skunk River, granite outcrops near 34 

Slate, distribution and character of.. 59, 60, 110 

use of term explained 3 

Sluicing at Rowe mine 103 

Snake River, granite outcrops near 47 

sandstone near c 

Split Rock River, rock outcrops near 67, 7 

Spurr, J. E., referred to 20,21,22,9 

Staples, granite outcrops near o 

Staurolite, occurrence of 62 

Stearns County, clay deposits in ?.. 94 

rock outcrops in 31, 38 

Structure, development of 11 1 

Subsilicic igneous rocks, distribution and 

character of 49 

Sultana mine 104 

analyses of rock from 120 

geology of 154 

production of 108 



Page 

Swan River, rock outcrops near 61 

Swan son, August, referred to. .101, 102, 105, 106 

Sweet, E. T., referred to 15 

Sweetman, E. A., acknowledgments to.... a 

Taylor's Falls, geology near 14 

Terminology a 

Thomas, Kirby, referred to 97.98,103 

Thompson, diabase outcrop near 89 

Thompson mine 101 

analysis of rock from. . 120 

geology of 148 

production of 108 

Thwaites, F. T., referred to 6,9,13,23 

Tod-Stambaugh Company 98, 142 

Todd, J. E., referred to 6 

Todd County, rock outcrops in 31,49 

Topography 95 

Torah ( Richmond), Alay near 94 

rock out crops ^«ar 39 

United States Geological Survey, coopera- 
tion of 1 

investigations by 15 

L'pham, Warren, referred to 

5, 6, 9, 18, 23, 29, 31, 35, 36, 
39, 40, 42, 43. 61, 64, 91. 9a, 94 

Van Hise, C. R., referred to 

....6, 10, 13, 17, 20, 22, 24, 
25. 31. 53. 63. 109. 124. «*9 

Vermilion district, geological report on.... 21 

Virginia Ore Mining Company 103, 166 

Vir aia slate, correlation of 28 

distribution and character of.. 12, 109, 112 

Wahkon, granite outcrops near 42 

Warman, granite quarry at 43 

Watab, granite outcrops near 38 

Weidman, S., referred to 5 

White Fish Lake, limestone reported near 63 

Wilcox mine 105 

geology of 164 

production of 108 

Williams' quarry 50 

Willow River, sandstone near 91 

Winchell, N. H., referred to 5, 6, 9. 

18, 21, 25, 29, 53, 64, 69, 92, 93 

Wisconsin, sequence of rocks in 5, 6 

Wolff, J. F., referred to 12 

Woodbridge, D. E., referred to 99. 10 1 

Zapffe, Carl, referred to 95. ><> 2 

104, no, in, 112, 123, 124, 154 




MAP AND SECTIONS SHOWING THE GEOLOGY AND WORKINGS OF THE 

THOMPSON MINE 

CUYUNA IRON-ORE DISTRICT 

Surveyed July, 1915 

LEGEND 




PLATE XX